Can I build an ADU on my lot?

An accessory dwelling unit (ADU) is a second, smaller home on a single-family lot: a backyard cottage, a garage apartment, a unit inside the house. Whether you can build one depends on state law, which in some states requires cities to allow ADUs; on the city’s own ADU rules; and on whether the unit fits within the lot’s setbacks and coverage.

Updated October 14, 2026. General information, not legal advice: the city’s code and its planning staff decide for your lot.

What counts as an ADU

An ADU has its own kitchen, bathroom and entrance. Codes usually tell apart a detached ADU (a building of its own), an attached one (an addition to the house) and a conversion (a garage, basement or attic made into a home). Some states also define a junior ADU inside the house. A guest room without a kitchen is usually not an ADU, and a second full-size house usually needs a district that allows two homes on a lot.

State law comes first

Several states require cities to allow ADUs and limit what cities may demand. Three examples, as of October 2026:

• California: cities must allow ADUs on lots with a single-family home, and local coverage, floor-area and open-space rules may not stop an ADU of 800 sq ft with 4 ft side and rear setbacks. A junior ADU of up to 500 sq ft inside the house is allowed too. The state’s housing department keeps the current rules and handbook (opens in a new tab).
• Washington: cities and counties planning under the Growth Management Act must allow two ADUs on each residential lot inside urban growth areas, and may not cap their size below 1,000 sq ft. The Department of Commerce explains the rules (opens in a new tab).
• Texas: there is no statewide ADU law (a 2025 bill passed the Senate and died in the House). Each city decides: some allow ADUs in single-family districts with their own size and setback rules, others only in some districts or not at all.

Oregon, Montana, Vermont and Maine, among others, have state ADU laws as well. Check your state’s housing or planning agency.

The city’s ADU rules

Look for an ADU section in the zoning code, or ADUs in the district’s list of allowed uses. It usually states:

• which districts allow an ADU, and whether by right or only with a special permit
• the largest size, in square feet or as a share of the main house
• height and setbacks, which can be smaller than the house’s for a detached unit
• whether the owner must live on the property, and how much parking is required
• whether the unit may be rented short-term (and almost always that it may not be sold separately)

Room on the lot

Where an ADU is allowed it still has to fit. A detached unit needs room inside the setbacks, and its footprint counts toward lot coverage.

Rules beyond zoning

An HOA or deed restriction can forbid a second home even where the city allows one, though some states, California among them, limit that. Utility connections, impact fees and the building code add cost, and in a flood zone the new unit has to meet the flood rules.

How to check your lot

1. Find your zoning district on the city’s zoning map.
2. Check whether your state has an ADU law, then read the city’s ADU section.
3. Work out the room: the setbacks, and the coverage cap minus what already stands.
4. Check the deed restrictions and HOA rules.
5. Ask the city’s planning desk to confirm before you pay for a design.

How the Lot Report helps

The Lot Report shows the district’s setbacks and limits, how much of them the existing buildings use where the record shows them, and whether the district’s rules point to a second unit. Where the data to decide that is missing it says “not known” instead of guessing.