How are ADUs built? Site-built, modular and more

Why does the building method matter?

Most ADUs get built one of five ways. A crew frames it on your lot, stands up walls made in a shop, sets finished boxes from a factory, delivers a federally certified manufactured home, or assembles a kit. The real difference is who checks the work, and when.

That choice changes how long your yard is a job site, how much you can customize, and what paperwork your town’s code office, the local office that issues building permits, wants before anyone moves in.

One thing stays the same. Zoning, your town’s rules for what you may build and where, comes first no matter how the unit is built. A well-built home can still be turned down if your lot doesn’t allow a second dwelling. Start with the property checker or your state page, such as New York’s, before you shop.

Tiny houses on wheels and park-model RVs follow their own rules, covered in our tiny house guide.

Built on your lot: site-built, panelized and kits

Site-built

A site-built ADU, sometimes called stick-built, is framed piece by piece on your property, usually in wood. It gives you the most design freedom. A crew can work around an odd-shaped lot, match the main house’s roofline and trim, or tie into existing walls. That is why garage apartments, basement units and additions are nearly always site-built.

The trade-off is time on site. Every step happens in order in your yard, and a Finger Lakes winter can slow framing and roofing. Your local building inspector checks the work in stages as it goes up.

Panelized

With panelized construction, wall, floor and roof sections are built in a shop, trucked in and stood up on your foundation. The shell closes in quickly, which helps in bad weather. Everything after that, from wiring to trim, is still done by a crew on site.

Panels come two ways. “Open” panels arrive as framing and sheathing, so an inspector can see inside the wall. “Closed” panels arrive with insulation, wiring or plumbing already sealed in.

In New York: the state’s factory-building rules cover buildings made off site in a way that can’t be inspected at the site without taking them apart (19 NYCRR 1209.1, NY Department of State). Closed panels may fall into that group. Ask your code office before you order them.

Kit homes

A kit is a package of pre-cut or pre-engineered parts, such as a timber frame, an A-frame or a panel set, that a builder or a handy owner puts together on a foundation. For code purposes it is a site-built house, with the same permit, plan review and inspections.

The two catches are climate and scope. A kit designed for a mild climate may not be engineered for our snow or our frost depth, which is how far down the ground freezes in winter. The Town of Perinton sets frost depth at 42 inches (Town of Perinton, 2025) and the Town of Ithaca lists 48 inches (Town of Ithaca, 2026). The kit price also tends to leave out the foundation, utilities, heating and labor. One kit seller says the finished cost often runs two to three times the kit price (Mighty Small Homes, 2026).

In New York: every city, town and village must publish its local design numbers, including ground snow load and frost depth (19 NYCRR 1203.3(k), NY Department of State). Get them from your code office and give them to the kit maker before you buy.

Modular: finished in a state-approved factory

A modular ADU is built as one or more finished boxes, called modules, in a climate-controlled factory. Walls, wiring, plumbing and often cabinets and finishes are done before it ships. A crane then sets it on a permanent foundation your builder has ready.

A modular home meets the same building code as a site-built one. What changes is where the checking happens. Because your builder can pour the foundation and run utilities while the factory builds the unit, the overall schedule can be shorter. The industry’s trade group says modular projects can finish 30% to 50% sooner, with 60% to 90% of the work done indoors (Modular Building Institute, 2026). That is the industry’s own estimate, not an independent study.

The limits are practical. A truck and a crane need a clear path into your yard, so narrow driveways, tight backyards and overhead wires can rule it out or add cost. Modules have shipping-size limits that shape the floor plan. Changes after your factory slot is booked are expensive, and factories usually take a deposit early.

In New York: every modular unit must carry a state Insignia of Approval showing it was built to the state’s Uniform Code and Energy Code (NY Department of State). The state approves each model’s plans, and a state-approved inspection agency checks units in the factory (19 NYCRR 1209.3). Your local inspector doesn’t reopen the walls, but does check the foundation, the utility hookups and local rules such as height and placement (19 NYCRR 1209.2).

HUD-code manufactured homes

A manufactured home is built entirely in a factory to one federal standard written by the U.S. Department of Housing and Urban Development (HUD). People used to call these mobile homes. Homes built since June 15, 1976 carry a HUD label showing they meet that standard (NY Department of State). Under federal law, a manufactured home is at least 8 feet wide or 40 feet long, or at least 320 square feet once set up (42 U.S.C. §5402, Cornell LII).

It is usually the lowest-priced way to get a complete, inspected home. The average new manufactured home sold for about $134,800 in April 2026, and about $133,900 in the Northeast (U.S. Census Bureau, 2026). Ask what a price like that leaves out: delivery, setup, the foundation, hookups and permits can add a lot.

For about 50 years, HUD-code homes had to sit on a permanent steel chassis (42 U.S.C. §5402, Cornell LII). A federal law that took effect July 11, 2026 made the chassis optional (Orrick, 2026). The law doesn’t override local zoning, so your town still decides where manufactured homes may go (Baker Botts, 2026).

Financing is the other thing to check. Loans that treat a factory home as personal property, like a car, are called chattel loans, and they tend to carry higher rates than mortgages (CFPB, 2021). Freddie Mac, one of the big buyers of home loans, wants a manufactured-home ADU titled as real estate (Freddie Mac, 2024). Talk to a lender before you buy.

In New York: a manufactured home installed after January 1, 2006 needs a building permit, and a state-certified installer has to be on site during installation (NY Department of State). Your local code officer inspects the installation but has no say over the factory construction, which HUD’s standard controls (same source).

How do the methods compare?

Real schedules depend most on permits and your builder’s backlog, so read the timeline column as relative.

Who checks the work? Codes and inspections in plain words

Two sets of rules apply to every ADU. Zoning decides what you may build and where: whether an ADU is allowed, how big it can be, and how far it sits from the property line. The building code decides how it must be built to be safe: structure, fire protection, ways out, insulation, plumbing and wiring.

In New York: the building code is the statewide Uniform Code, enforced by your town’s code office. Its 2025 edition, which includes the residential code, took effect December 31, 2025 (NY Department of State, 2025). Every new dwelling needs a building permit. The exemption towns may adopt for sheds and playhouses up to 144 square feet never covers a place people live (19 NYCRR 1203.3(a)(1)).

For a site-built, panelized or kit ADU, New York’s rules for local code offices set out the basic path (19 NYCRR Part 1203):

• Permit and plans. You or your builder submit drawings of the structure, exits, insulation and systems, plus a site plan based on a survey of your lot lines.
• Inspections in stages. The inspector comes at set points, such as footings and foundation, framing, building systems, insulation and a final. Work has to stay uncovered until it passes.
• Certificate of occupancy. This is the paper saying the finished work passed. No one may legally live in the ADU until the code office issues it.

Factory-built homes follow a split path: the plant work is checked under the state’s modular program or HUD’s system, and your local inspector checks the foundation, hookups and installation. Before issuing a certificate of occupancy for a factory-built home, the code office must confirm the required insignia, seals and data plates are attached (19 NYCRR 1203.3(d)).

Two more code items catch people off guard:

• Attached ADUs. An ADU that shares a wall or floor with the main house turns the building into a two-family dwelling. New York’s code then calls for walls and floors between the units rated to hold back fire for 1 hour, or half an hour if the whole building has sprinklers (2025 Residential Code of New York State, R302.3, via UpCodes).
• All-electric rules. New York enacted restrictions for covered new construction, with exceptions for alterations, additions and changes of use to buildings existing before the applicable prohibition (NYS Energy Law §11-104). As checked October 9, 2026, the Department of State says the adopted fossil-fuel provisions remain suspended by court order; other 2025 code provisions remain in effect (DOS notice). We have not confirmed when the suspension will end. Ask your code office which current rules apply to your project before choosing equipment.

Our permits guide walks through the steps, and site and utilities covers foundations, septic and power.

What this means for you

• Settle zoning first. Pick a method once you know your town allows the ADU you want, in the spot you want it.
• Attached units and conversions are usually site-built. Anything tied into the main house needs a crew on site.
• Factory routes trade flexibility for site time. Modular and manufactured homes spend less time in your yard, if a truck and crane can get in and you can commit to a design early.
• Read the label before the price. A New York insignia means modular, built to the state code. A HUD label means manufactured. An RV seal means neither, and our tiny house guide explains why that matters.
• Compare full prices. Get quotes at the move-in level, with foundation, delivery, hookups and permits included. Our cost guide shows what goes into one.
• Bring in the right people. An architect or engineer for plans, a builder who knows your town’s inspectors, and a lawyer or lender for title and loans. Nothing here is legal or financial advice.

Questions to ask any supplier

Ask these of any builder, factory or kit company before you sign.

• Which code is this built to, and what label will it carry: a New York insignia, a HUD label, or neither?
• Who inspects it in the factory, and what will my local inspector still need to see?
• Has a code office near me accepted your plans before? Can you provide plans sealed by a New York-licensed architect or engineer if my town asks?
• Is it engineered for my town’s snow load, wind and frost depth?
• What is the turnkey price, meaning everything needed to move in? What does your quote leave out, such as foundation, delivery, crane, hookups or permits?
• Can a truck and crane reach my site, who checks that, and who pays if they can’t?
• How much is the deposit, when is it due, and what happens to it if my permit is delayed or denied?
• Can the finished unit be titled as real estate and financed with a regular mortgage?
• What warranty covers the work, and who fixes a problem after the unit is set?

General information, not legal, tax or financial advice. Your municipality decides what can be built, and a professional should review decisions about money and contracts.