What Is an Attached ADU?

An addition that is its own home

An attached ADU starts as a home addition, then crosses a line: it gets a separate door, a full kitchen and a bathroom, and no interior path that a tenant must use through the main house. Built that way, it is a second dwelling unit under the zoning code, not just extra rooms.

California’s definition of an accessory dwelling unit names both attached and detached units, and § 66314(d)(3) lists the attached forms: attached to, or located within, the primary dwelling, including attached garages and storage areas ².

In practice, planners separate three cases: new floor area added to the house (attached), existing rooms converted (an interior ADU), and a small unit inside a single-family home (a junior ADU).

Key takeaways

• An attached ADU is a full second home joined to the main house by at least one wall.
• It needs its own door, kitchen and bathroom. Extra rooms that open only into the house do not count.
• In California the state standard caps it at half the size of the existing home ².
• But a city cannot use that half rule to block an 800 sq ft unit ³.
• A city must allow it to be up to 25 ft tall, or the local limit for the house if that is lower ³.
• Shared walls mean fire and sound separation, and work on the main house.

Why the attached label matters to you

The type you pick sets your size limit. For a detached unit, California uses a fixed cap. For an attached unit, the cap is tied to the size of your house ². So two neighbors with the same lot can get very different numbers.

The label also changes the work. An attached unit joins your roof, your walls and often your foundation. That can save on utility runs. It can also mean upgrades to the old house that a detached unit would never touch.

Last, it changes daily life. You share a wall with the people next door. Good sound and fire separation is not a nice extra. It is what makes two homes feel like two homes.

The 50% rule and the 800 sq ft floor (California)

Two sections set the size, and they pull in opposite directions. § 66314(d)(4) says that if there is an existing primary dwelling, the total floor area of an attached ADU shall not exceed 50 percent of it ².

§ 66321(b)(3) then says a city may not use any size limit "based upon a percentage of the proposed or existing primary dwelling", or lot coverage, floor area ratio, open space, front setback or minimum lot size rules, to prevent an ADU of at least 800 sq ft of interior livable space with four-foot side and rear setbacks ³.

Read together: half the house is the cap, except that on a small house the cap cannot drop below 800 sq ft. Separately, § 66321(b)(2) stops a city from setting any maximum below 850 sq ft, or 1,000 sq ft for a unit with more than one bedroom ³. Your city’s ordinance applies these; the planning department makes the call.

The formula puts both rules on one line. The worked example below runs it on three house sizes.

Sources: [2] [3]

Kinds of attached ADU

Worked example: three house sizes

Applying the state sections above to three hypothetical existing homes. A city may allow more; it may not allow less than the protected 800 sq ft.

The decision rule: on a home under 1,600 sq ft the 800 sq ft floor, not the 50% figure, sets what you can count on. Check your own numbers with the size envelope calculator.

Attached vs detached vs junior ADU

How an attached ADU project works

1. Measure the house. Find the floor area of the existing home. In California the 50% cap starts here ².
2. Pick where it goes. Side, rear or up. Each one meets a different setback or height rule.
3. Check the old structure. A designer or engineer looks at the roof, walls and foundation that the new unit will touch.
4. Plan the separation. Fire-rated walls and floors, and sound control, go on the plans. The building department sets the details.
5. Plan the entrances. The unit needs its own way in from outside.
6. Apply for the permit. In California the city reviews it against fixed rules, with no hearing ⁴.
7. Build and inspect. Expect the main house to be part of the job site for a while.

Where "attached" shows up on paper

• On the permit form, as the ADU type. The box you tick decides the size rule.
• On the floor plan, where the shared wall is marked with its fire rating.
• On the site plan, which shows the distance to the side and rear lot lines.
• On the area table, which shows the existing home’s floor area next to the new unit’s area. That is how a plan checker tests the 50% rule.
• On a builder’s bid, as tie-in work: roof, framing and finishes where old meets new. Compare bids with the bid comparison tool.

Rules outside California

Costs and what drives them

There is no one fair price for an attached ADU, so this page gives none. The cost depends on what you build and on the house you join it to.

The main drivers are size, the number of kitchens and baths, and how much the old house must change. A second story may need new support below it. A rear addition may need a new foundation. Shared walls need fire-rated build-ups.

Fees matter too. In California, a unit of 750 sq ft or less carries no impact fee. Above that, impact fees must be in proportion to the size of the main home ⁵. See sourced cost data on the ADU cost hub and try the ADU cost calculator.

Why build attached, and what to watch

Common mistakes

Myth Any addition with a bathroom is an attached ADU.

Reality No. It needs full living facilities, including a kitchen, and its own entrance. Otherwise it is just more rooms in the house ¹.

Myth Half the house is always the limit.

Reality Not in California. On a smaller home the 800 sq ft floor wins over the 50% rule ³.

Myth An attached unit is always cheaper.

Reality Not always. Tying into an old house can cost more than a simple new building. Compare real bids.

Myth A city can require a hearing for it.

Reality Not in California. ADU review is ministerial, with no hearing ⁴.

What to ask before you build

• What does the city count as the existing home’s floor area?
• Is my house small enough that the 800 sq ft floor applies?
• What fire rating does the shared wall or floor need?
• Can the existing foundation and framing carry a second story?
• Will the work trigger upgrades to the main house, such as the electrical panel? See electrical service upgrades.
• Does the unit need its own utility meters? See separate utility meter.

Checklist for an attached ADU

1. Find the floor area of your existing home on old permits or a new measure.
2. Work out 50% of it, and compare with 800 sq ft.
3. Check your city’s own maximum for one bedroom and for two.
4. Mark side and rear setbacks on a lot sketch.
5. Have the old structure looked at before design.
6. Ask for fire and sound separation on the plans.
7. Plan a separate outdoor entrance.
8. Get at least two bids and compare the tie-in work line by line.

Related guides and tools

• ADU types hub
• ADU type selectorAttached, detached or conversion?
• ADU regulations hub
• ADU size statistics
• ADU cost drivers

Related terms

Accessory Dwelling Unit

An accessory dwelling unit (ADU) is a second, self-contained home on a lot that already has, or will have, a main house or apartment building, with its own space to live, sleep, cook, eat and wash.

Detached ADU

A detached ADU is an accessory dwelling unit built as its own freestanding building on the same lot as the main home, usually in the backyard, with its own kitchen, bathroom and entrance.

Junior ADU

A junior ADU (JADU) is a small home of no more than 500 square feet carved out of an existing or proposed single-family house, with its own entrance and an efficiency kitchen, and permission to share the main home’s bathroom.

ADU Size

ADU size is the floor area of an accessory dwelling unit, usually counted as interior livable square feet, and it is the number that state and local rules cap, protect and use to set fees.

Converted ADU

A converted ADU is an accessory dwelling unit made from space that already exists, such as a garage, basement, attic, spare wing or shed, instead of a new building.

Height Limit

A height limit is the maximum vertical height a zoning code allows for a building, measured from a reference point on the ground to a reference point on the roof that the local code defines.

Setback

A setback is the minimum distance a zoning code requires between a building and a property line, measured horizontally from the line to the nearest wall or other regulated part of the structure.

Floor Area Ratio

Floor area ratio (FAR) is the total floor area of all buildings on a lot, on every story, divided by the lot’s area, so a FAR of 0.5 allows 3,000 square feet of building on a 6,000 square foot lot.

Also referenced from: Accessory Dwelling Unit, Detached ADU, Above-Garage ADU, Basement ADU, Junior ADU, Converted ADU, Two-Story ADU, ADU Size