ADU types: which kind fits your home?

What is an interior conversion?

An interior conversion turns part of your house into a separate apartment. Many are basements; some are attics, a back wing or a floor of a larger home. The walls, the roof and much of the plumbing are already there, and the footprint (the ground the house covers) doesn’t grow, so lot coverage limits rarely come into it.

That head start usually makes it one of the cheapest types. In a California survey, conversions inside the house had a median cost of $100,000, about $173 per square foot, for projects permitted in 2018 and 2019 (UC Berkeley Center for Community Innovation, 2021). A national cost guide puts basement apartments at $60,000 to $150,000 (Angi, 2026).

The catches are about safety and comfort:

• Headroom. Living spaces normally need ceilings at least 7 feet high (Fine Homebuilding, 2024). Under a sloped attic roof, floor where the ceiling is lower than 5 feet doesn’t count toward a room’s required size (2025 Residential Code of New York State, §R312.3).
• Escape windows. Every bedroom needs an opening big enough to climb out of in a fire: at least 5.7 square feet of clear opening, 24 inches tall and 20 inches wide, with the sill no more than 44 inches off the floor (Boulder County, 2021 IRC §R310). A small, high basement window almost never qualifies.
• Fire separation. A second home inside the house makes it a two-family dwelling, so the walls and floors between the units need a 1-hour fire rating, or ½ hour with sprinklers (International Code Council, 2021 IRC §R302.3).
• Moisture, radon and sound. Basements need dry walls and floors and a radon test. Footsteps carry through shared floors, so plan sound insulation between the levels.

In New York: When a basement or attic in an existing house becomes living space, the 2025 residential code lowers the ceiling minimum to 6 feet 8 inches, and to 6 feet 4 inches for bathrooms and under beams or ducts (2025 Residential Code of New York State, §R313.1.2). Some towns allow only this type in certain areas. In the Town of Clarkson’s RS-10 district, an ADU must be inside the house, with its entrance on the side or rear (Town of Clarkson Code §140-23.4).

What is an attached ADU?

An attached ADU is a new addition built onto the house as a separate home, with its own entrance, kitchen and bathroom. It shares at least one wall with the main house, and that wall backs onto heated space instead of the outdoors.

Built at ground level with no step at the door, an addition suits a parent who may use a walker or wheelchair later, with help a doorway away. Because it is new construction, it has to fit your setbacks (the minimum distances from your property lines) and your lot coverage limit. The shared wall must meet the same fire-separation rule as an interior unit (International Code Council, 2021 IRC §R302.3).

Small additions cost a lot per square foot, since a kitchen, a bathroom and a foundation cost about the same at any size (UC Berkeley Center for Community Innovation, 2021). Angi puts attached ADU additions at $100,000 to $216,000 (Angi, 2026).

In New York: Towns often control how an addition connects to the house and how it looks from the street. The Village of Fairport requires an attached ADU to share at least 25% of its wall area, or its floor or ceiling, with the house, and the addition can’t extend past the front of the existing house (Village of Fairport Code §551-26).

What is a detached backyard cottage?

A detached ADU is its own small house in the yard. It gives both households the most privacy and is the easiest type to build all on one level, with no step at the entry.

It usually costs the most, because it needs its own foundation, roof, outside walls on every side and utility lines run out from the house. In the California survey, detached units had a median cost of $180,000, about $300 per square foot, compared with $90,000 for garage projects (UC Berkeley Center for Community Innovation, 2021). Angi lists new detached units at $110,000 to $285,000 before permits and fees (Angi, 2026).

Size follows local caps more than anything else. New ADUs in California averaged 615 square feet (UC Berkeley Center for Community Innovation, 2021), and Seattle’s ADUs permitted from 2016 to 2021 had a median of 750 square feet (City of Seattle, 2023).

Check these early:

• Frost and foundations. Footings must sit below the frost line or use an insulated shallow design, and water and sewer lines need protection from freezing too.
• Septic and well. If you’re not on public sewer, adding a home can mean a bigger septic tank or field, and the field has to keep its distance from any well.
• Distance from the house. Homes built close together need fire-resistant walls where they face each other (2025 Residential Code of New York State, §R302.1).
• Trenching and access. Long runs for power, water and sewer add cost, and a truck or crane needs a path if any part is built off site.

In New York: Frost depth is set town by town; the Town of Perinton requires footings 42 inches down (Town of Perinton, 2025). Septic systems are sized at 110 gallons per bedroom per day for homes with low-flow fixtures (10 NYCRR Appendix 75-A.3), and a leach field must sit at least 100 feet from a well (10 NYCRR Appendix 75-A.4). Town rules swing widely: the Village of Fairport bans detached ADUs (Village of Fairport Code §551-26), while the Town of Canandaigua lists them as a permitted use in several districts (Town of Canandaigua Code Ch. 220).

In New York: 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.

What about garages?

A garage offers two routes.

Converting the garage. The walls and roof already stand, which usually makes this one of the cheaper paths. Angi’s 2026 average for turning a garage into an ADU is $110,000, with typical jobs from $60,000 to $150,000 and a New York average of $136,000 (Angi, 2026). The slab and walls usually need insulation and moisture control, and the big door opening becomes a wall with windows. A detached garage also needs new underground utility lines, which Angi says adds 20% to 30% (Angi, 2026). You lose the parking, too, and some towns require room for an extra car when you add an ADU (Town of Clarkson Code §140-23.4).

Building an apartment over the garage. A new second story adds a home without adding footprint, which helps on tight lots. It comes with stairs, so it’s a poor fit for anyone who struggles with them. Sloped ceilings can shrink the usable space, since floor under a ceiling lower than 5 feet doesn’t count toward a room’s minimum size (2025 Residential Code of New York State, §R312.3). The garage may need structural work to carry the new floor, and that floor needs full insulation over the cold space below. Angi lists above-garage units at $128,000 to $225,000 (Angi, 2026).

What is a junior ADU?

A junior ADU, or JADU, is a California legal category: a unit of 500 square feet or less inside a single-family house, which may share a bathroom with the main home (California Government Code §66313). Picture a bedroom suite with a small kitchen and its own outside door.

New York has no JADU category, and outside California the term mostly turns up in articles and ads. A suite that shares the family bathroom generally isn’t a separate home under the building code, which expects each dwelling unit to have its own toilet, sink, tub or shower and kitchen sink (2025 Residential Code of New York State, §R326). In New York it’s simply an in-law suite in your house. Ask your town how it treats one before you plan to rent it out. Angi puts converting a finished room into an in-law suite at $10,150 to $65,625 (Angi, 2026).

How do the types compare?

Every type needs a building permit, inspections and a final sign-off before anyone moves in (19 NYCRR §1203.3). The zoning approvals on top of that depend on your town, so the column below shows what tends to apply, not a rule.

The California figures are for projects permitted in 2018 and 2019 (CCI, 2021), and building costs have risen since, so read them as a ranking rather than a price list. The pattern holds: the more new structure you add, the more you pay. How the unit gets built is a separate choice; How are ADUs built? compares site-built, modular and panelized paths.

Which type fits your goal?

Start with the person, then check what the lot and the town allow.

In New York: Converting a building to a different use requires the appropriate occupancy or compliance certificate. A permit for storage does not approve later housing (19 NYCRR §1203.3(d)).

If anyone might grow old in the unit, build it accessible from day one. Federal accessible-design rules cover buildings with four or more units, so for a house plus an ADU it’s your choice (24 CFR §100.201). A step-free entry, wide doors and wood blocking in the bathroom walls for future grab bars cost little on paper and a lot to add later. Building an ADU for aging in place covers the details.

What this means for you

• Pick the person first, then the type, then the size.
• Conversions reuse what you have and usually cost least. New detached cottages cost most but live best on one level.
• Every type needs a building permit and a final inspection. Detached units tend to draw the most zoning review, and some towns don’t allow them at all.
• Basements and attics live or die on headroom and escape windows. Measure before you pay for plans.
• “Junior ADU” is a California term. In New York, a suite that shares the family bathroom is part of your house, not a second home.
• Your town decides which types are allowed on your lot. Call your town’s code office, read How to check your town’s rules, or start with the property checker. Bring in an engineer for structure, septic or a tricky lot, and a lawyer for deeds or rental questions.

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.