What you can build

Turning a basement into a legal apartment (ADU)

Checked against state laws and official agency sources · Last reviewed October 10, 2026

Is a basement apartment an ADU?

Usually, yes, if it is a complete home of its own. State laws define an ADU by what it contains, not by which floor it's on:

• California: an ADU "has complete independent living facilities for one or more persons" and can be detached, attached, or "located within the proposed or existing primary dwelling."
• Colorado: an ADU is "an internal, attached, or detached dwelling unit" with "facilities for living, sleeping, eating, cooking, and sanitation."
• Utah: an "internal accessory dwelling unit" is one created within the home and "within the footprint of the primary dwelling."
• Washington: an "attached accessory dwelling unit" is one "located within or attached to" a house, duplex, triplex, townhome or other housing unit.

In California, a smaller unit inside a single-family home can instead be a junior ADU: no more than 500 square feet of interior livable space, "contained entirely within a single-family residence." It may have its own bathroom or share one with the house. See our junior ADU guide.

Seattle notes that ADUs "are not legal unless they have been established through a permit process." A basement apartment built without one is an unpermitted unit, even if it has been rented for years. [1][4][5][6][10]

State laws that cover basement conversions

California. Converting existing space in your home into an ADU is not subject to unit size limits. No setback can be required for an ADU made within existing living area. Objective standards on height, lot coverage, landscape and design in a local ADU ordinance don't apply to conversions. If you dig a new basement for the ADU, HCD treats it as an attached ADU, which can't exceed 50 percent of the existing home's floor area. On a lot with an apartment building, the city must allow ADUs in space not used as living space, such as storage rooms, attics, basements or garages: at least one, and up to 25 percent of the existing units.

Utah. In any area zoned primarily for residential use, "the use of an internal accessory dwelling unit is a permitted use." The city may not restrict its size compared with the house, lot size, street frontage or internal connectivity. It may still bar one on a lot of 6,000 square feet or less, in a home on a failing septic tank, in a mobile home, or in certain zones, and it may require an extra parking space. The unit "shall comply with all applicable building, health, and fire codes."

Washington. Cities and counties must allow at least two ADUs on lots in urban growth areas where single-family homes are allowed, including "two attached accessory dwelling units." They can't set a maximum ADU size below 1,000 square feet. State law counts basements in a home's "gross floor area."

Colorado. Since June 30, 2025, covered cities and counties "shall allow, subject to an administrative approval process, one accessory dwelling unit as an accessory use" to a single-unit detached home wherever such homes are allowed.

See our California, Utah, Washington and Colorado pages for the details, and check your city's page before you plan the work. [1][2][3][4][5]

What building codes usually require

Building codes for houses are adopted by each state and often amended by cities, so the exact numbers depend on where you live. Oregon, for example, amended its 2023 residential code to keep its earlier escape-opening rule instead of the new model code language. The rules below come from official code text and city handouts. Treat them as examples and confirm the rules with your building department.

Emergency escape openings. Oregon's residential code says: "Basements, habitable attics and every sleeping room shall have not less than one operable emergency escape and rescue opening. Where basements contain one or more sleeping rooms, an emergency escape and rescue opening shall be required in each sleeping room." The opening must lead to a public way, or to a yard or court at least 36 inches wide that opens to one. Exceptions include basements used only for mechanical equipment and no larger than 200 square feet. In a home with an NFPA 13D sprinkler system, basement bedrooms don't each need one if the basement has two code-compliant exits, or one exit plus one escape opening.

Escape opening size. Los Angeles's ADU bulletin, based on California's 2025 Residential Code, requires escape openings at least 24 inches high, 20 inches wide and 5.7 square feet open, with the bottom no more than 44 inches above the floor.

Ceiling height. Los Angeles requires "a minimum ceiling height of 7 feet" throughout an ADU conversion, and habitable rooms of at least 70 square feet. In Portland, Oregon, new elements must meet the state code, but existing elements in an existing building can use the city's habitable space guide: flat ceilings at least 6 feet 8 inches, with beams, ducts and pipes allowed as low as 6 feet within 2 feet of a wall, or 6 feet 2 inches if they cover no more than 10 percent of the room. Portland doesn't allow covenants to waive ceiling height for an ADU.

Portland's basement allowances. Under the same guide, an existing window used for escape must have its opening no more than 44 inches above the floor, and a single permanent step up to 12 inches high can be used to reach that. In basements, an exterior door at least 6 feet 2 inches high and 30 inches wide can serve as the emergency exit. Existing basement stairs that can't meet the guide must be rebuilt or walled off if another compliant exit is provided. [7][8][9][11]

Fire separation, alarms and sprinklers

• Fire separation. Portland requires dwellings to be "separated from each other and common space by wall and floor assemblies having not less than a one-hour fire-resistance rating," with sound ratings of 45 STC and 45 IIC for floor/ceiling assemblies. For conversions, existing walls and ceilings covered with lath and plaster or half-inch drywall in sound condition can stay, if they were permitted and passed final inspection more than two years before the ADU application.
• Doors between units. In Portland, a door between a dwelling and shared space must be at least 1-3/8-inch solid wood, honeycomb-core steel or 20-minute fire-rated.
• Smoke alarms. Los Angeles requires smoke alarms "in each sleeping room and immediately outside of each separate sleeping area," at least 20 feet from a permanently installed cooking appliance (with listed exceptions). Its checklist for unpermitted units looks for smoke alarms on every level, "including basements."
• Carbon monoxide alarms. The same checklist looks for carbon monoxide alarms outside each sleeping area, "on every occupiable level including basements," and in bedrooms with fireplaces.
• Sprinklers. In California, fire sprinklers are not required in an ADU or junior ADU if the main home isn't required to have them.

[1][7][9]

Plumbing, heating and utilities

Portland allows an ADU to share the main home's sanitary drainage system, and says "the most common example of this condition is when the ADU is located in the basement." If the ADU's floor is lower than the next uphill manhole cover on the sewer, each new fixture's drain needs a backwater valve. Drains from fixtures above the basement can't run through those valves.

Basements often hold the furnace and water heater. Portland requires separate heating systems for the ADU and the main home, but in a conversion the main home's furnace may stay where it is if the main home can reach it directly or through a common space. Existing ducts serving the main home may stay in the ADU if they are sealed where they enter it, and exposed ducts below the ADU's ceiling are wrapped in 5/8-inch Type X gypsum board. Fuel-fired equipment in the converted area must still get enough combustion air.

Los Angeles requires a separate electrical panel and water shutoff for an ADU. Portland requires electrical circuits "completely independent of" the main home's.

In California, an ADU made from existing space in your home can't be required to have a new or separate utility connection, unless it is built along with a new single-family home. Utah lets cities bar a separate utility meter for an internal ADU. [1][2][7][9]

How to check whether a basement apartment is legal

Ways to find out whether a basement unit was permitted:

• Ask for the permit records. Boulder, for example, tells owners who are unsure of a space's established use to review its Planning & Development record resources or submit a records request.
• Look for a certificate of occupancy. In California, you must get a certificate of occupancy from the city or county before anyone lives in an ADU or junior ADU.
• Ask the city how the unit is listed. Seattle calls a legally permitted unit in the home an attached ADU, and says legalizing an existing unit takes "a construction permit to establish use."

If the unit was never permitted, see our unpermitted ADU guide for legalization paths, including California's rules for units built before 2020. [1][6][12]

Permit steps for a basement conversion

The general steps for any ADU are in our guide to the ADU permit process.

Some cities name the permit. Seattle says that adding an ADU within an existing house needs "a construction addition / alteration permit." When Boulder legalizes an existing unit, it requires stamped plans from a licensed design professional, and finishes such as drywall and tile will need to come off so inspectors can see the framing, insulation, plumbing, mechanical and electrical work. [6][12]

Questions people ask