An attached ADU is new construction connected to your existing home, usually at the side or rear. It suits lots where a separate backyard building will not fit, and it can share some structure and utility runs with the house. It needs its own entrance and full living facilities.
How big can an attached ADU be?
As of October 2026, state law defines an attached ADU as one that does not exceed 50% of the floor area of the existing home. Cities must still allow at least 850 sq ft, or 1,000 sq ft with more than one bedroom, so a small house does not shrink the ADU below that floor. In the City of Los Angeles, City Planning's ADU guidance also caps attached ADUs at 50% of the home's existing living area. Your zone's floor-area rules can apply on top.
How tall can an attached ADU be?
As of October 2026, cities must allow an attached ADU up to 25 feet, or the height limit that applies to the main house, whichever is lower. That can make a second-story ADU over part of the house possible on some lots. A city may set a more generous limit.
What setbacks apply?
As of October 2026, cities cannot require more than 4-foot side and rear setbacks for an ADU. Front setback rules can apply where they do not block an 800 sq ft unit. Where the house already sits close to a side line, the addition's shape follows whatever space the setbacks leave open.
Are there impact fees on an attached ADU?
As of October 2026, ADUs of 750 sq ft or less of interior livable space are exempt from impact fees under state law. Larger ADUs can be charged fees in proportion to the size of the main home. Permit and plan-check fees still apply.
Attached ADU or junior ADU?
Both connect to your home. A junior ADU is built inside existing space and is limited to 500 sq ft. An attached ADU adds new square footage, can be larger, and carries no owner-occupancy rule. If you want more than 500 sq ft or a fully separate unit, attached is usually the better fit.
What does an attached ADU depend on?
- The existing house. Its foundation, roof lines and wall locations shape where the addition can tie in.
- Fire separation. The shared wall and ceiling need the separation the building code requires between units.
- Sound and privacy. Layout and insulation decide how independent the two homes feel.
- Utilities. Whether your electrical service and sewer line can serve both units.
We give you a written price after the site visit. We do not publish a typical attached ADU price here because we could not find reliable Los Angeles-specific figures as of October 2026.