What you can build

How many ADUs can you have on one property?

Checked against state laws and official agency sources · Last reviewed November 5, 2026

ADUs per lot by state

What each state's law makes cities (and in some states counties or towns) allow on a lot. These are minimums: a city can allow more. Where the law covers only some places, the table says which. Details, quotes and sources are on each state page.

Not listed? We don't cover your state yet. Ask your city or county planning department. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42]

California: ADU plus junior ADU combinations

California's law lists the units a city must approve "or any combination" of them on a lot with a single-family home (existing or proposed):

• One ADU and one junior ADU inside the existing or proposed space of the house. The ADU can instead go inside the existing space of an accessory building (up to 150 square feet can be added, only for getting in and out).
• One new detached ADU. The city can't require side and rear setbacks of more than 4 feet. It can cap the ADU at 800 square feet of livable space and limit its height.

California's housing agency (HCD) sums it up: on single-family lots, cities "must allow at least one ADU constructed from existing space, one JADU, and one newly constructed detached ADU." That is three extra units on one lot. Our junior ADU guide explains what a junior ADU is.

From January 1, 2027: AB 956, signed September 29, 2026, raises the detached ADUs a city must approve on a single-family lot from one to two, each still capped at 800 square feet if the city chooses. A city then doesn't have to approve a junior ADU on a lot where two new detached ADUs have been built under that rule. California statutes passed in a regular session take effect on January 1 after a 90-day period from enactment. [5][6][7][19]

Single-family vs. multifamily lots

Most state ADU laws are written for lots with a single-family home. California also covers lots with apartment buildings (multifamily lots):

• ADUs inside the building. A city must allow at least one ADU within an existing multifamily building, in space not used as living space, such as storage rooms, attics, basements or garages, and must allow up to 25 percent of the existing units.
• Detached ADUs. On a lot with an existing multifamily building, up to eight detached ADUs, but no more than the number of existing units on the lot. On a lot with a proposed multifamily building, up to two.

Maine's law, from July 1, 2027, also extends ADUs to lots with a 2-unit or 3-unit building. Elsewhere, whether you can add an ADU to a duplex or apartment lot is usually up to your city. [5][19][28][29]

Cities that allow more than the state minimum

State laws set floors, not ceilings. Some cities we cover allow more:

• Seattle: up to two ADUs per lot. The code says "No lot may have more than two accessory dwelling units" and they "may be attached, detached, or stacked."
• Austin: Texas has no statewide ADU law, but Austin allows up to three housing units, including tiny homes, on lots zoned SF-1, SF-2 and SF-3, "attached or detached in any combination."
• St. Paul: Minnesota has no statewide ADU law, but St. Paul allows two ADUs for each one-family home on a lot. The city's example: a lot with two one-family homes can have up to four ADUs. If you have two ADUs, at least one must be detached, unless you keep at least 50% of the floor area of the existing house.

Check your city's page on our states and cities list or ask your city planning department. [43][44][45]

Lot splits and second homes: SB 9 and similar laws

An ADU is an extra unit on the same lot as a main home. Some laws let you add a second main home instead, or split the lot.

In California, SB 9 lets an eligible lot be split into two, in HCD's words "with each of the two resulting lots eligible for up to two primary units." After a split, the city doesn't have to allow more than two units on either new lot, ADUs included. Without a split, HCD says an eligible lot can get up to two homes under SB 9 plus the ADUs and junior ADUs it could otherwise have. If a lot is split and also uses SB 9's two-home rule, the city doesn't have to allow any ADU or junior ADU on it. See our SB 9 lot split guide.

In Maine, from July 1, 2027, towns must allow at least 3 homes per lot where homes are allowed (4 in growth areas or on public water and sewer), counting ADUs. [19][29][46][47]

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