Guides

ADU glossary

157 terms you’ll meet when planning an ADU, in plain words. Terms marked NY are specific to New York.

Above-garage ADU: A home built as a second floor over a garage. Stairs and sloped ceilings can cut the usable space and make it hard for some people to live there. See also: Ceiling height, Detached ADU, Garage conversion.
Absorption field: The part of a septic system where liquid from the tank soaks into the ground through buried trenches; people also call it a leach field or drain field. In New York, it must sit at least 100 feet from a well and from any stream, lake or wetland. See also: Percolation test, Septic system, Septic tank, Well.
Accessory apartment: Another name some local codes use for an ADU, often for a unit inside or attached to the main house. When you search your town's code, look for both "accessory apartment" and "accessory dwelling unit," since each code defines its own words. See also: Accessory dwelling unit (ADU), ecode360.
Accessory dwelling unit (ADU): A second, smaller home on the same lot as a main house, with its own kitchen, bathroom, sleeping space and door. It can be inside the house, attached to it, or in its own building in the yard. See also: Attached ADU, Detached ADU, Dwelling unit.
Accessory structure: A building on your lot that no one lives in, like a garage, shed or workshop. A backyard office with no kitchen is usually one of these, not an ADU, so different rules apply. See also: Accessory use, Dwelling unit.
Accessory use: A use of land that is secondary to the main use and allowed only alongside it, like a private garage on a house lot. Some codes, such as the Village of Pittsford's, treat an ADU as an accessory use to a single-family home. See also: Permitted use, Special use permit.
Adjacent area NY: The strip of land around a state-regulated freshwater wetland that New York also protects. It is usually 100 feet wide, and building, grading or septic work there can need a permit from the state DEC. See also: DEC (NYS Department of Environmental Conservation), Freshwater wetland, Jurisdictional determination.
Agricultural data statement NY: A form that goes with some zoning requests, like a special use permit, when your land is near a farm. It's needed if your land is in, or within 500 feet of, a farm in a county agricultural district, and the town then notifies the nearby farm owners. See also: County referral (239-m review), Special use permit.
All-Electric Buildings Act NY: A 2023 New York law that directs state codes to restrict fossil-fuel equipment in most new buildings, with exceptions. As checked October 12, 2026, the state Department of State still lists the implementing provisions as suspended and unenforceable. Existing-building work, including additions and changes of use, is treated separately. Confirm the requirements and timing for your project with your local code office. See also: Heat pump, NYS Clean Heat.
Appraisal: A licensed expert's written opinion of what a property is worth, usually ordered by a lender. Where few homes with ADUs have sold nearby, the appraisal may not give full credit for what you spent to build one. See also: As-completed value, Gross floor area.
Area median income (AMI): The middle household income for an area: half of households earn more and half earn less. Grant programs often set income limits as a share of it; New York's Plus One ADU grants have used 120% of AMI. See also: Fair Market Rent (FMR), Plus One ADU Program.
Area variance NY: Permission from the zoning board of appeals to break a size rule, such as a setback or height limit. In New York the board weighs five factors, including harm to the neighborhood and how big the ask is. See also: Use variance, Variance, Zoning board of appeals (ZBA).
Article 78 proceeding NY: The type of lawsuit used in New York to ask a court to review a local board's decision. A challenge to a town special use permit must be filed within 30 days after the decision is filed. See also: Public hearing, Special use permit.
As-completed value: An appraiser's estimate of what your property will be worth after planned work, such as a new ADU, is finished. Renovation mortgages use this number to set how much you can borrow. See also: Appraisal, Renovation mortgage.
Assessed value NY: The value your town puts on your property to figure your tax bill. In New York it is market value times the town's level of assessment, so a new ADU will usually raise it. See also: Grievance Day, Level of assessment, RPTL 421-p.
Attached ADU: An ADU that shares a wall, floor or ceiling with the main house, like an addition or a basement unit. New York's code treats this as a two-family home, which brings fire-safety rules. See also: Detached ADU, Fire separation, Two-family dwelling.
Base flood elevation (BFE): The height floodwater has a 1% chance of reaching in any year, shown on FEMA flood maps. In New York, the lowest floor of a new building in a flood zone must be at least 2 feet above it. See also: Floodplain development permit, Special flood hazard area.
Basement ADU: An apartment built in a home's basement. Ceiling height, an escape window big enough to climb through, dampness and radon are the usual hurdles. See also: Ceiling height, Egress window, Radon.
Blower-door test: A test that uses a big fan in a doorway to measure how much air leaks out of a building. New York's 2025 code limits new homes in zone 5 to 3.0 air changes per hour at the test pressure. See also: Energy code, Mechanical ventilation.
Boundary survey: A map by a licensed land surveyor that shows your exact property lines, buildings and recorded easements. New York building permits need a site plan based on one. See also: Easement, Setback, Site plan.
BP-1 form NY: A New York form that lets a homeowner get a building permit without a contractor's insurance proof. It's only for owners who live in the home and act as their own builder, within strict limits. See also: Owner-builder, Workers' compensation proof.
Building code: The rules for how a building must be built to be safe, covering structure, fire protection, exits, insulation, plumbing and wiring. It is separate from zoning, which decides what you may build and where. See also: International Residential Code (IRC), Uniform Code, Zoning.
Building permit: Written permission from your town's code office to start work. In New York every home needs one, however small; only minor work like sheds of 144 square feet or less can be exempt. See also: Certificate of occupancy (CO), Code enforcement officer (CEO), Inspection.
By right: Allowed without a hearing or a board's judgment call, as long as you meet the written rules. Staff must approve plans that follow the code; this is also called "as of right." See also: Permitted use, Special use permit.
Call before you dig (811, UDig NY) NY: A free service that marks buried utility lines before you dig; outside New York City and Long Island it's run by UDig NY. Contact it 2 to 10 working days ahead, and know it won't mark private lines you own, like a wire to a garage. See also: Easement, Septic system.
Cash-out refinance: Replacing your mortgage with a bigger one and taking the difference in cash. It resets the interest rate on your whole loan balance, not just the new money, so it can cost more than a second loan if your current rate is low. See also: HELOC (home equity line of credit), Home equity loan.
Ceiling height: The lowest ceiling the code allows in a room. New York requires 7 feet in living spaces and halls and 6 feet 8 inches in bathrooms, with lower limits for some basement and attic conversions. See also: Basement ADU, Habitable space, Tiny house.
Certificate of occupancy (CO): The paper from your town's code office saying finished work passed inspection and may be used; a short-term "temporary CO" is sometimes possible. In New York, no one may legally live in a new ADU until one is issued. See also: Building permit, Inspection.
Chattel loan: A loan that treats a factory-built home as personal property, like a car, instead of as real estate. The federal consumer bureau found these loans tend to carry higher rates than mortgages. See also: Manufactured home, Real property.
CHOICERenovation: Freddie Mac's renovation loan. Its list of allowed work includes adding or fixing up an ADU. See also: Freddie Mac, HomeStyle Renovation, Renovation mortgage.
Climate zone: A region the energy code groups by how cold it gets, which sets insulation and window rules. Most of the Rochester and Finger Lakes area, including Monroe and Ontario counties, is in zone 5. See also: Energy code, R-value, U-factor.
Code enforcement officer (CEO) NY: The local official who reviews building permits, inspects the work and issues the certificate of occupancy. New York towns, villages and cities do this job unless they hand it to the county or the state. See also: Building permit, Uniform Code, Zoning board of appeals (ZBA).
Code office: The town, village or city office that handles zoning questions and building permits, inspects the work and issues the certificate of occupancy; some places call it the building department or code enforcement office. Call it before you pay for drawings. See also: Building permit, Certificate of occupancy (CO), Code enforcement officer (CEO).
Comprehensive plan NY: A town's long-range plan for how it wants to grow, covering land use, housing and services. Under New York law, a town's zoning and other land-use rules must agree with its adopted comprehensive plan. See also: Zoning, Zoning district.
Condominium conversion: Splitting one property into units that can be sold separately. California lets cities allow this for ADUs, but we found no statewide path to do it in New York. See also: Subdivision.
Construction drawings: Scaled plans that show exactly how a building will be built, including structure, insulation, plumbing, wiring and a site plan. The code office reviews them before it issues a permit. See also: Site plan, Stamped drawings.
Construction loan: A short-term loan that pays for a build in stages, called draws, as each phase is finished and inspected. When the work is done, it is paid off with a new loan or turned into a regular mortgage. See also: Contingency, Renovation mortgage.
Contingency: Money set aside in your budget for surprises. Federal renovation loans treat a reserve of 10% to 20% of the work as normal, and older homes and conversions deserve the high end. See also: Turnkey price.
County referral (239-m review) NY: A rule that gives the county planning office 30 days to weigh in on some zoning requests. It applies within 500 feet of a town line, a state or county road, or a farm in an agricultural district. See also: Agricultural data statement, Site plan review, Special use permit.
DEC (NYS Department of Environmental Conservation) NY: The New York agency that regulates freshwater wetlands, streams, stormwater and some flood rules. Its free online Environmental Resource Mapper is a good first check for wetlands near your lot. See also: Adjacent area, Freshwater wetland, Stormwater.
Deed restriction: A private rule in a property's records, like "one house only" or "no rentals"; it's also called a restrictive covenant. It can block an ADU even where zoning allows one, so read your deed before you design. See also: Easement, HOA (homeowners association).
Depreciation: A yearly tax deduction for the wear on a rental building. Residential rental buildings (not the land) are generally depreciated over 27.5 years, and that deduction can be taxed back when you sell. See also: Home sale exclusion, Schedule E.
Design flow NY: The amount of wastewater per day a septic system must be built to handle. New York figures it by bedrooms, using 110 gallons per bedroom per day for homes with modern low-flow fixtures. See also: Septic system, Septic tank.
Detached ADU: An ADU in its own building, such as a backyard cottage or a converted detached garage. A new detached unit usually costs the most of any ADU type because it needs its own foundation, roof and utility lines. See also: Attached ADU, Fire separation, Garage conversion.
Dimensional rules: The measurement limits in a zoning code, such as setbacks, height, lot coverage, minimum lot size and maximum unit size; some codes call them bulk rules. Codes often list them in a table for each zoning district. See also: Lot coverage, Setback, Zoning district.
Dwelling unit: A home that gives one household complete, independent living, with permanent space to live, sleep, eat, cook and wash. New York's 2025 residential code requires each one to have a toilet, a sink, a tub or shower, and a kitchen area with a sink. See also: Accessory dwelling unit (ADU), Accessory structure.
Easement: A right someone else has to use part of your land, such as for a power line or a shared driveway. You usually can't build on top of one, and a survey will show any that are on record. See also: Boundary survey, Deed restriction.
ecode360 NY: A website where many New York towns and villages post their local laws, including zoning. Check its "legislation through" date, since newer laws may not be in the text yet. See also: Accessory apartment, Zoning.
Egress window: A window big enough and low enough to climb out of in a fire; below ground it needs a window well. New York requires one in every bedroom, with at least 5.7 square feet of opening and a sill no higher than 44 inches. See also: Basement ADU, Habitable space.
Energy code NY: The rules for how well a building holds in heat and keeps out drafts. New York's 2025 energy code took effect December 31, 2025, with no grace period. See also: Blower-door test, Climate zone, R-value.
Enhanced treatment unit NY: A septic part that cleans wastewater better than a plain tank. Under New York's 2025 rules, a maintained unit can make the drain trenches about a third shorter, outside the New York City watershed. See also: Absorption field, Septic system.
Fair housing laws: Laws that bar landlords from treating renters differently because of race, religion, sex, disability, children or other traits. New York covers more groups than federal law, including renters who pay with housing vouchers. See also: Lease, Security deposit.
Fair Market Rent (FMR): HUD's yearly estimate of a typical modest rent, including utilities, for a given area and unit size. Some ADU grant programs cap rents at this level. See also: Area median income (AMI), Regulatory agreement.
Fannie Mae: A government-sponsored company that buys home loans from lenders and sets the rules those loans must follow. Its rules now let a borrower count part of the rent from one ADU as income on a purchase or limited cash-out refinance. See also: FHA loan, Freddie Mac, Qualifying income.
FHA 203(k) loan: An FHA mortgage that rolls the cost of repairs or an addition into one loan, and FHA's list of eligible work includes adding an ADU attached to the house. A smaller "Limited" version covers lighter jobs up to a dollar cap that FHA reviews each year. See also: Contingency, Renovation mortgage.
FHA loan: A mortgage insured by the Federal Housing Administration, which is part of HUD. Since October 2023, FHA lets lenders count part of an ADU's rent as income in many cases, but not on cash-out refinances. See also: FHA 203(k) loan, Qualifying income.
Fire separation: Walls, floors and ceilings built to slow a fire's spread from one home to another. In an attached ADU in New York, the two units must be separated by construction rated for 1 hour, or half an hour if the whole building has sprinklers. See also: Attached ADU, Setback, Two-family dwelling.
Floodplain development permit NY: Permission from your town's floodplain office, usually the building or zoning office, to build in a mapped flood zone. Building in a floodway, where a flood's main flow runs, also needs an engineer's study. See also: Base flood elevation (BFE), Special flood hazard area.
Footprint: The ground a building covers, measured to the outside of its walls. Zoning uses it to figure lot coverage, and the space inside the walls is always somewhat smaller. See also: Gross floor area, Lot coverage.
Foundation: The base that holds a building up and ties it to the ground, such as a concrete slab, a crawlspace, a full basement, or piers and screw-in steel helical piles. A soils (geotechnical) report helps pick the right type on lots with clay, fill, slopes, rock or wet spots. See also: Frost depth, Frost-protected shallow foundation (FPSF).
Freddie Mac: A company, like Fannie Mae, that buys home loans and sets lending rules. It lets part of an ADU's rent count as income and wants a manufactured-home ADU titled as real estate. See also: CHOICERenovation, Fannie Mae, Real property.
Freshwater wetland NY: Land that is wet often enough to support wetland plants and soils, like a marsh or swamp. Since January 1, 2025, New York regulates wetlands of 12.4 acres or more, or of unusual importance, whether or not they appear on state maps, and the size cutoff drops to 7.4 acres on January 1, 2028. See also: Adjacent area, DEC (NYS Department of Environmental Conservation), Jurisdictional determination.
Frost depth: How far down the ground freezes in winter, which sets how deep footings must go so frost can't lift the building; it's also called the frost line. The Monroe County towns of Perinton and Chili require 42 inches, and the Town of Ithaca lists 48 inches. See also: Foundation, Frost-protected shallow foundation (FPSF).
Frost-protected shallow foundation (FPSF): A shallow slab with foam insulation around its edges that keeps the ground below from freezing. New York allows it only for buildings kept at 64°F or warmer on average, so it may not suit an ADU left unheated. See also: Foundation, Frost depth.
Garage conversion: Turning an existing garage into an ADU. It is often one of the cheaper paths because the shell already stands, though the floor, walls, insulation and utilities usually need work. See also: Above-garage ADU, Attached ADU, Detached ADU.
Good Cause Eviction NY: A 2024 New York law that, where it applies, makes landlords give a legal reason to evict or not renew a lease and makes big rent increases hard to justify. Outside New York City it applies only where a city, town or village opts in, as Rochester and Ithaca have. See also: Lease, Owner occupancy requirement.
Grievance Day NY: The day each year when you can formally challenge your property assessment before your town's board of assessment review. It is usually the fourth Tuesday in May, though dates vary. See also: Assessed value, Taxable status date.
Gross floor area: The total floor area of a building, usually measured to the outside of the walls. Towns define it differently, so ask what counts toward your ADU size limit; appraisers use a similar measure called gross living area. See also: Appraisal, Footprint.
Habitable space: Rooms used for living, sleeping, eating or cooking; bathrooms, closets, halls and storage don't count. New York's code says habitable rooms other than kitchens must be at least 70 square feet and at least 7 feet across in every direction. See also: Ceiling height, Dwelling unit.
Hamlet NY: A named place in New York with no government of its own. The town around it handles zoning and building permits. See also: Town, Village.
Heat pump: A machine that heats and cools by moving heat with electricity instead of burning fuel. Ductless "mini-split" models suit small ADUs, and ENERGY STAR's cold-climate label means a model was tested at 5°F. See also: All-Electric Buildings Act, NYS Clean Heat.
HELOC (home equity line of credit): A credit line backed by your home, where you borrow as needed during a draw period and pay interest only on what you've used. The rate usually floats, and the lender can freeze the line if your home's value drops, which could stall a build. See also: Cash-out refinance, Home equity loan.
Historic district: An area a local government protects for its history, where changes to how buildings look may need a design review first. New York law lets towns, villages and cities create these rules. See also: Overlay district, Zoning district.
HOA (homeowners association): A private group that runs a neighborhood and enforces its own rules, which can limit ADUs or rentals. A few states override HOA bans on ADUs, but we found no New York law that does. See also: Deed restriction.
Home equity loan: A fixed-rate second mortgage that pays you one lump sum, borrowed against the part of your home you own outright. You pay interest on the full amount from the start. See also: Cash-out refinance, HELOC (home equity line of credit).
Home improvement contract NY: The written contract New York requires for most home improvement jobs over $500, though it may not cover a new detached ADU. It must list the work, the dates, a payment schedule and your right to cancel within 3 business days. See also: Turnkey price.
Home sale exclusion: A federal tax break that lets you skip tax on up to $250,000 of profit ($500,000 for most married couples filing jointly) when you sell a home you owned and lived in for 2 of the last 5 years. A separate rented unit, like a detached ADU, may not fully qualify. See also: Depreciation, Schedule E.
HomeStyle Renovation: Fannie Mae's renovation mortgage, where the cost of the work can be up to 75% of the home's as-completed value. Ask the lender whether it will finance a new detached ADU, since we found no guide language naming one. See also: CHOICERenovation, Renovation mortgage.
HUD code: The federal building standard for manufactured homes; homes built to it since June 15, 1976 carry a HUD label. In New York, local inspectors check only the installation, not the factory work. See also: Manufactured home, Modular home.
Impact fee: A one-time charge some towns put on new homes to help pay for roads, schools or water and sewer capacity. ADU fees vary by town, so ask yours whether one applies. See also: Tap fee.
Insignia of Approval NY: The New York State label on each modular unit showing it was built to state code under factory inspection. Your local inspector trusts it for the factory work and checks the foundation and hookups. See also: Modular home, Uniform Code.
Inspection: A building inspector's visit at set stages, like footings, framing, wiring, insulation and final. In New York, work must stay uncovered until it passes. See also: Building permit, Certificate of occupancy (CO).
International Residential Code (IRC): A model building code for one- and two-family homes, written by the International Code Council for states to adopt and change. New York's residential code is an amended version of it. See also: Building code, Uniform Code.
JADU (junior accessory dwelling unit): A California legal term for a unit of 500 square feet or less inside a house that may share a bathroom. New York has no such category, so you'll mostly see it in California articles. See also: Accessory dwelling unit (ADU), Attached ADU.
Jurisdictional determination NY: A letter from the state DEC saying whether a wetland on or near your land is regulated by the state. DEC has 90 days to answer, and the letter stays valid for 5 years. See also: Adjacent area, Freshwater wetland.
Kit home: A package of pre-cut building parts shipped to your lot to be put together. Kit prices often leave out the foundation, utilities, heating and labor, so the finished cost usually runs well above the kit price. See also: Panelized construction, Prefab, Turnkey price.
Landlord policy: Insurance for a home you rent out that covers the building, your liability and lost rent after damage. A homeowners policy usually won't cover a rented unit, and tenants need renters insurance for their own things. See also: Lease, Short-term rental.
Lease: A written contract between a landlord and tenant that sets the rent, how long the tenant stays and the house rules. For an ADU, it should also say who uses which parts of the yard and driveway, who clears snow, and how utilities are split. See also: Fair housing laws, Good Cause Eviction, Security deposit.
Legal nonconforming: A building or use that was legal when built but doesn't meet today's rules; people call it "grandfathered." It can usually stay, and Freddie Mac accepts such ADUs for its loans. See also: Variance, Zoning.
Level of assessment NY: The single percentage of market value a New York town uses to assess every property. For example, a $100,000 home in a town assessing at 27% would have a $27,000 assessment. See also: Assessed value.
Lot coverage: The share of a lot that buildings, and sometimes pavement, may cover under zoning. A detached ADU's footprint usually counts, so on small lots this limit, more than lot size, often decides what fits. See also: Dimensional rules, Footprint.
Manufactured home: A factory-built home made to the federal HUD code that is at least 8 feet wide or 40 feet long, or 320 square feet once set up. Since July 11, 2026, federal law no longer requires a steel chassis under it. See also: HUD code, Modular home, ROAD to Housing Act.
Mechanical ventilation: A fan system that brings in fresh air on purpose, often one that saves heat from the stale air going out (an HRV or ERV). New York requires it in homes sealed tight enough to meet the energy code. See also: Blower-door test, Energy code.
Medicaid look-back period: The 60 months before a nursing home Medicaid application when gifts of assets, like a share of a home, are checked and can delay coverage. Some transfers to a caregiving child who lived there are exempt, so see an elder law attorney first. See also: Owner occupancy requirement.
Modular home: A home built as finished boxes in a factory to the same code as a site-built home, then trucked in and set on a foundation by crane. In New York each unit must carry a state Insignia of Approval. See also: Insignia of Approval, Manufactured home, Prefab.
NYS Clean Heat NY: New York's heat pump rebate program, run through utilities such as RG&E and NYSEG. Its 2026 manual says brand-new buildings can't get rebates for air-source heat pump heating, though existing homes can, so check before you count on one. See also: All-Electric Buildings Act, Heat pump.
Off-street parking requirement: A zoning rule that you provide a set number of parking spaces on your own lot instead of on the street. Some codes require an extra space for an ADU, as the Village of Pittsford does. See also: Dimensional rules.
Overlay district: An extra zoning layer with added rules for places like lakefronts, steep slopes or flood areas. Its rules apply on top of your base district's rules. See also: Historic district, Zoning district.
Owner occupancy requirement: A local requirement that whoever owns the lot make one of the two units, house or ADU, their home. It can stop you from renting both units if you move, and it can shrink the pool of future buyers who hope to borrow against rental income. See also: Good Cause Eviction, Short-term rental.
Owner-builder: A homeowner who acts as their own general contractor and runs the job. It can save money but shifts the risk to you, and some loans and permit forms limit how much of the work you can do yourself. See also: BP-1 form, Workers' compensation proof.
Panelized construction: Building with wall, floor and roof sections made in a shop and stood up on site. "Open" panels leave the wiring, plumbing and insulation for the site crew, while "closed" panels arrive with some of that sealed inside. See also: Kit home, Modular home, Prefab.
Park model RV: An RV of up to 400 square feet built for seasonal or short-term living. Federal rules say it isn't meant as a permanent home, so it usually can't be a legal year-round ADU. See also: Tiny house on wheels.
Percolation test: A test, often called a perc test, that measures how fast water soaks into the soil so a septic field can be sized. New York requires at least two for each septic system. See also: Absorption field, Septic system.
Permitted use: A use a zoning district allows outright, without a special permit. The Town of Canandaigua, for example, allows a house plus one ADU this way in several districts. See also: Accessory use, By right, Special use permit.
Planning board NY: An appointed local board that usually reviews site plans and subdivisions and, in many towns, special use permits. The town board decides which board handles which kind of application. See also: Site plan review, Special use permit, Zoning board of appeals (ZBA).
Plus One ADU Program NY: A New York State grant that helps income-eligible homeowners build or fix up an ADU, with up to $125,000 per unit outside New York City. You apply through a local partner, and at our last check in October 2026 none served Monroe or Ontario County. See also: Area median income (AMI), Regulatory agreement.
Pre-application meeting: An informal early meeting with town planning staff or the planning board to talk through an idea before you file. We found no state rule requiring one, but asking for one can save time and money. See also: Planning board, Site plan review.
Pre-approved plans: Standard ADU designs a local government has already reviewed, so homeowners who use them can get permits faster. New York City launched a set in 2026; ask whether your town has anything similar. See also: Building permit, Construction drawings.
Prefab: Any home built partly or fully in a factory. It covers modular, manufactured, panelized and kit homes, which follow different rules, so ask which one a seller means. See also: Kit home, Manufactured home, Modular home, Panelized construction.
Public hearing: A meeting where a board takes comments from neighbors and the public before deciding an application. For a New York town special use permit, notice must be mailed at least 10 days ahead and published in a local paper at least 5 days ahead. See also: Article 78 proceeding, Special use permit.
Qualifying income: The income a lender counts when deciding how big a mortgage you can handle. FHA, Fannie Mae and Freddie Mac can add part of an ADU's rent, generally 75% of it, but cap the ADU rent at 30% of your total qualifying income. See also: Fannie Mae, FHA loan, Freddie Mac.
R-value: A number for how well insulation slows heat loss; higher is better. New York's 2025 code calls for R-49 attics in zone 5, and some walls add "continuous" foam outside the studs. See also: Climate zone, Energy code, U-factor.
Radon: A natural radioactive gas from the ground that can build up indoors and cause lung cancer. The EPA recommends fixing a home that tests at 4 picocuries per liter or higher. See also: Basement ADU.
Real property: Land and anything fixed to it, like a house on a foundation, unlike personal property such as a car. Freddie Mac requires a manufactured-home ADU to be titled as real property. See also: Chattel loan, Manufactured home.
Regulatory agreement: A contract that comes with some housing grants and makes the owner follow rules, such as rent limits or no short-term rentals, for a set number of years. New York's Plus One program uses agreements of at least 10 years. See also: Fair Market Rent (FMR), Plus One ADU Program.
Renovation mortgage: A home loan that includes money for planned work and sets the loan amount on the home's value after the work. The main types are FHA 203(k), Fannie Mae HomeStyle and Freddie Mac CHOICERenovation. See also: As-completed value, Construction loan.
Reverse mortgage: A loan for homeowners 62 or older that needs no monthly payment; the balance grows and is usually repaid when the home is sold. The federally insured version is called a HECM, and it's wise to talk to a HUD-approved housing counselor first. See also: Home equity loan.
ROAD to Housing Act: A federal housing bill, the 21st Century ROAD to Housing Act, that became law on July 11, 2026. It dropped the steel-chassis rule for manufactured homes and tells HUD to write model local ADU rules. See also: HUD code, Manufactured home.
RPTL 421-f NY: A local-option New York tax break for improving a one- or two-family home at least five years old. It exempts up to $80,000 of added value, phasing out over eight years, and may or may not cover an ADU. See also: Assessed value, RPTL 421-p.
RPTL 421-p NY: A local-option New York tax break, created in 2024, for building a new ADU. Where adopted, it can exempt up to $200,000 of added value, fully for five years and then less each year until year 10. See also: Assessed value, Taxable status date.
Schedule E: The federal tax form for reporting rent and rental costs, including depreciation. See also: Depreciation, Home sale exclusion.
Security deposit: Money a tenant pays up front to cover unpaid rent or damage. In New York it can't be more than one month's rent, and you must return it within 14 days after move-out with an itemized list of any deductions. See also: Lease.
Septic system: An underground system that treats a home's wastewater on its own lot: solids settle in a tank, and liquid soaks into the soil through a field. New York sizes systems by the number of bedrooms, so an ADU's bedrooms can mean a bigger tank or field. See also: Absorption field, Design flow, Septic tank.
Septic tank: The buried tank where solids settle out of wastewater before the liquid flows to the field. In New York, homes with one to three bedrooms need at least a 1,000-gallon tank and four bedrooms need 1,250 gallons. See also: Design flow, Septic system.
SEQR NY: New York's environmental review, which checks whether a board's decision could harm the environment; sometimes you fill out a short form for it. Many homeowner approvals, like setback variances, are "Type II" and skip review. See also: Type II action.
Service size: How much power, in amps, your utility line and main panel can deliver; 100 and 200 amps are common. An electrician does a load calculation to see if it can carry an ADU, which is often fed by a subpanel from the house. See also: Submeter.
Setback: The minimum distance a building must stay from a property line, street or other building. Zoning sets most setbacks, and the building code adds fire-safety rules for walls close to a line. See also: Boundary survey, Dimensional rules, Fire separation.
Short-term rental: Renting a home to guests for short stays, usually less than 30 days in a row, often through sites like Airbnb or Vrbo. In New York, sales tax has applied to these stays since March 1, 2025, and towns can still require permits or ban them. See also: Owner occupancy requirement, Short-term rental registry.
Short-term rental registry NY: A county list of short-term rental hosts set up under a New York law passed in 2024 and amended in 2025, with safety and insurance rules for hosts. Counties could opt out, and Monroe County voted to do so in December 2025. See also: Short-term rental.
Site plan: A scaled drawing of your lot that shows property lines, buildings, driveways, utilities, and any well or septic system. See also: Boundary survey, Construction drawings, Site plan review.
Site plan review NY: A planning board's review of where a project sits on the lot, plus the driveway, parking and landscaping. In New York towns, any hearing must come within 62 days of applying, and the decision within 62 days after. See also: Planning board, Special use permit.
Snow load: The weight of snow a roof must be designed to hold, which each town sets. The Town of Ithaca, for example, lists a ground snow load of 40 pounds per square foot, with more required at higher elevations. See also: Frost depth.
Special flood hazard area: Land FEMA maps as likely to be covered by the "base flood," the flood with a 1% chance of happening in any year. If you build there with a federally backed or regulated loan, flood insurance is required. See also: Base flood elevation (BFE), Floodplain development permit.
Special use permit NY: Approval for a use the code allows only if a board finds your project meets set conditions and won't harm the neighborhood. In New York towns, the hearing must come within 62 days of applying and the decision within 62 days after. See also: Article 78 proceeding, Planning board, Public hearing.
Stamped drawings NY: Construction drawings signed and sealed by a licensed architect or engineer. New York law doesn't require a seal for a home of 1,500 square feet or less (not counting garages, porches, or unfinished basements and attics), but your town may still ask for one. See also: Construction drawings.
Stormwater: Rain and snowmelt that runs off roofs and driveways; it can't go into a sewer or septic system. In most of New York, a state permit is needed only if you disturb an acre or more, but towns may want a drainage plan. See also: DEC (NYS Department of Environmental Conservation).
Subdivision: Dividing one lot into two or more legal lots. An ADU usually can't be sold apart from the main house unless the lot is split this way or turned into condos, and some codes forbid a separate sale outright, as the Village of Pittsford's does. See also: Condominium conversion.
Submeter: A meter an owner adds behind the utility meter to track one unit's use. In New York, billing tenants for power this way needs state approval first, which can take 6 to 8 months. See also: Service size.
Tap fee: A one-time charge to hook a new building up to public water or sewer. Water authorities and sewer districts set their own fees, and some charge per unit, so ask whether your ADU counts as a new one. See also: Impact fee.
Taxable status date NY: The date each year that fixes a property's condition and ownership for assessment, and usually the deadline to apply for exemptions. It is March 1 in most New York communities. See also: Assessed value, RPTL 421-p.
Tiny house: A very small permanent home on a foundation. New York's 2025 residential code has a tiny-house appendix for homes of 400 square feet or less, not counting lofts, with eased rules for ceiling height and ladders or steep stairs to sleeping lofts. See also: Ceiling height, Tiny house on wheels.
Tiny house on wheels: A small home built on a trailer. Whether it can be a legal year-round home depends on the standard it was built to and on local zoning, and many are certified as RVs, which aren't meant for permanent living. See also: Park model RV, Tiny house.
Town NY: One of New York's main types of local government. A town handles zoning and building permits for its land outside any village. See also: Hamlet, Village.
Turnkey price: A price that covers everything needed to move in, including foundation, delivery, hookups and finishes. A factory unit-only price can be well under half the turnkey price, so compare every quote at this level. See also: Contingency, Kit home, Prefab.
Two-family dwelling: A building that holds two separate homes. A house with an attached ADU usually counts as one under New York's residential code, which brings fire-separation rules between the units. See also: Attached ADU, Fire separation.
Type II action NY: A kind of project New York's rules say needs no environmental review, like a one- to three-family home on an approved lot or a setback variance. The board decides whether a given ADU permit fits. See also: SEQR.
U-factor: A number for how much heat a window lets escape; the lower the number, the better. New York's 2025 energy code requires windows rated 0.27 or lower in climate zone 5. See also: Climate zone, R-value.
Uniform Code NY: New York's statewide building and fire code, which applies everywhere except New York City. Its 2025 edition, which includes the state's residential code, took effect December 31, 2025. See also: Code enforcement officer (CEO), Energy code, International Residential Code (IRC).
Universal design: Designing a home so people of all ages and abilities can use it, with things like a step-free entry, wide doors and a curbless shower. Federal access rules cover buildings with four or more units, so for most ADUs these features are your choice. See also: Ceiling height, Habitable space.
Use variance NY: Permission from the zoning board of appeals for a use the district doesn't allow, like an ADU where none are allowed. It's hard to win, because you must prove "unnecessary hardship," including that no allowed use earns a fair return. See also: Area variance, Variance, Zoning board of appeals (ZBA).
Variance: An official exception to a zoning rule, granted by the zoning board of appeals. New York has two kinds with very different tests: area variances for measurements and use variances for uses. See also: Area variance, Use variance, Zoning board of appeals (ZBA).
Village NY: A self-governing community inside one or more New York towns; its people vote and pay taxes in both. A village can have its own zoning, so its ADU rules may differ from the town's. See also: Hamlet, Town.
Well: A drilled or dug hole that supplies water from underground. New York's health department suggests a new home well reliably give at least 5 gallons per minute where possible, and wells that give less may need a storage tank. See also: Absorption field, Septic system.
Workers' compensation proof NY: Proof of workers' comp and disability insurance, or a signed exemption, that a New York town must see before it issues a building permit. See also: BP-1 form, Building permit.
Zoning: Local rules that decide what can be built on a piece of land, where on the lot it can go, how big it can be and how it can be used. In New York each city, town and village writes its own, because the state has no broad law making ADUs legal everywhere. See also: Building code, Comprehensive plan, Zoning district.
Zoning board of appeals (ZBA) NY: The local board that grants variances and hears appeals of the zoning officer's decisions. In a New York town, you have 60 days after a zoning officer's decision is filed to appeal it to the ZBA. See also: Area variance, Use variance, Variance.
Zoning district: An area on the zoning map with its own allowed uses and size rules. Your first ADU question is which district your lot is in and what it allows. See also: Dimensional rules, Overlay district, Permitted use.

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