The short answer
An attached ADU shares one or more walls with your existing house but has its own entrance, kitchen and bathroom. It suits deep, narrow Los Angeles lots where the back yard cannot take a free-standing unit, and it costs less than a detached build because it borrows the existing foundation edge, roofline and utility service.
Typical cost in Los Angeles
$400 to $600 per sq ft
Typical 600 sq ft attached unit: $240,000 to $360,000 all-in
An attached ADU shares one or more walls with your existing house but has its own entrance, kitchen and bathroom. It suits deep, narrow Los Angeles lots where the back yard cannot take a free-standing unit, and it costs less than a detached build because it borrows the existing foundation edge, roofline and utility service. State law allows up to 50 percent of the main house floor area.
When attached beats detached
- Narrow lot where a four-foot rear setback leaves no usable footprint
- You want to keep the back yard as yard rather than build on it
- The existing house already has a side wall facing unused side-yard space
- Budget is tight and sharing a wall and utility service saves 15 to 20 percent
- Housing a family member who needs to be close but independent
How it gets built
- New foundation tied into the existing footing with a structural connection detail
- One-hour fire separation between the ADU and the main dwelling
- Independent exterior entrance, never through the main house
- Roof tie-in detailed to match the existing pitch and material
- Separate electrical subpanel and its own water shutoff
Size limits in Los Angeles
- Up to 50 percent of the existing main house floor area
- Minimum 800 sq ft is always allowed regardless of that percentage
- Four-foot side and rear setbacks apply to the new portion
- Height matches the main house, subject to the 16-foot base limit
- Front-yard placement is not permitted
Get a straight answer on your lot in 48 hours
Send the property address. We pull the parcel data, zoning, setbacks and any overlay, then tell you whether a attached adu works on it, what size fits and roughly what it costs. No site visit needed and no obligation.
Free 48-hour lot check
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Common questions
How big can an attached ADU be?
Up to 50 percent of your existing house's floor area. If your house is 1,400 sq ft, that is 700 sq ft. You are always allowed at least 800 sq ft even when the percentage would give you less.
Can it share a door with the main house?
No. An attached ADU must have its own independent exterior entrance. An interior connecting door is allowed as long as the independent entrance exists and the fire separation is maintained.
Is it cheaper than a detached ADU?
Usually 15 to 20 percent cheaper per square foot, because the shared wall, shared roof plane and short utility runs cut both material and labor. The gap narrows on complex roof tie-ins.
Does it need a separate gas and water meter?
Not required in Los Angeles. Most owners keep shared service and sub-meter the ADU, which is cheaper than pulling a new service from the street. We wire for either.
Will it affect my property taxes?
The ADU is assessed as new construction and added to your base. The existing house is not reassessed. Talk to the LA County Assessor for the number specific to your parcel.
Related ADU types
Comparing options? A garage conversion is the cheapest path to a legal unit, a detached ADU earns the most, and the JADU stack can put both on one lot. Cost figures for every type are in the LA cost breakdown.
Areas we build in
Sherman Oaks / Van Nuys / Pasadena / Glendale / Burbank / Santa Monica / Long Beach / Torrance / North Hollywood / Studio City / Woodland Hills / Encino / Northridge / Culver City / Inglewood / Silver Lake and across Los Angeles County.