Attached ADU vs. Detached ADU: How They Compare.

Both are new construction, and both are real, separate homes. The difference is simple. An attached ADU shares a wall, floor or roof with your house. A detached ADU is its own building in the yard. That one difference changes the size rules, the height rules, the structure and often the plan.

LA ADU Guide is an information site. We do not design or build ADUs. Confirm details with your building department.

A note on law numbers: California's ADU law was renumbered by SB 477, effective March 25, 2024, into Gov. Code sections 66310 to 66342. Older articles cite section 65852.2.

Side by side

Treat the table as the state floor. Los Angeles and other agencies may allow more, but not less than state law requires.

Size

ABAG's July 2025 summary says a city with an ADU ordinance must allow at least 850 square feet (studio or one bedroom) or 1,000 square feet (two or more bedrooms) for both types. Only a city with no ADU ordinance falls back to 1,200 square feet for a detached ADU and 50 percent of the main home for an attached one. Section 66321(b)(3) says a city cannot adopt a rule that "does not permit an accessory dwelling unit with at least 800 square feet of interior livable space," so even a small main home can support an ADU of that size.

Cities may set maximum sizes of "850 square feet" for a one-bedroom unit or "1,000 square feet of interior livable space" for a unit with more than one bedroom, under section 66321(b)(2). These are the smallest maximums a city may set, so ask what your agency allows.

Height

Attached ADUs get the taller floor. Section 66321(b)(4) says a city must allow "a height of 25 feet or the height limitation in the local zoning ordinance that applies to the primary dwelling." Detached ADUs start at 16 feet, with 18 feet near transit and two more feet to match the roof pitch of the main home. HCD told the City of Los Angeles in 2024 that this can reach 20 feet.

That makes an attached ADU the easier way to build a second story. See two-story and garage ADUs.

Setbacks

For both types, state law says a city may not require more than four feet from the side and rear lot lines for the new unit described in section 66323(a). Front setbacks cannot block an ADU of at least 800 square feet and 16 feet in height that meets the four-foot rule (AB 2221, 2022). The practical difference is placement. An attached unit uses the footprint of the house and usually sits where the house already sits. A detached unit needs open yard and a path for utilities, access and fire safety.

Where each one tends to fit

These are general design points, not rules:

• Attached can work when the lot is narrow or short on yard, the house has a side or rear wall that can take an addition, or you want shared access. It can also keep utility runs short.
• Detached can work when you want more separation, a back-yard entrance, or the layout of the house makes an addition hard.

Neither is always cheaper. We do not have a sourced cost comparison for Los Angeles. Cost depends on the site, the structure and the finishes. See ADU cost in Los Angeles for what drives price.

Things that work differently

• Sprinklers. CAL FIRE says "All ADUs, attached or detached, require fire sprinklers if the primary dwelling is equipped with a fire sprinkler system." An addition cannot trigger sprinklers in the main home.
• Utilities. For units other than conversions, a utility may require a separate connection with a proportionate fee (section 66311.5(e)). See impact fees.
• Fees. No impact fee may be charged on an ADU of 750 square feet or less (section 66311.5(c)). Larger units pay proportionally to the main home.
• Fire separation and structure. An attached unit may need a rated wall and a coordinated structure with the main house. Ask the plan checker what applies.
• Review time. Both types get the same 60-day clock for a complete application (section 66317).
• Owner occupancy. Not required for either (AB 976, 2023).

Attached versus a JADU

A junior ADU is not an attached ADU. A JADU is at most 500 square feet and sits entirely inside the existing single-family home. An attached ADU is an addition. See ADU vs. JADU vs. garage conversion.

Checklist for choosing

• [ ] Measure your yard after setbacks, easements and trees.
• [ ] Find out your main home's square footage, since the attached cap is 50 percent of it.
• [ ] Ask the agency for the height limit for each type on your lot.
• [ ] Ask whether the main home has fire sprinklers.
• [ ] Ask whether your lot is in a fire, hillside or overlay area.
• [ ] Decide whether you want a shared wall, shared utilities, or separation.
• [ ] Get the same scope priced both ways, with a written estimate.

State rules appear in California ADU rules. More in the ADU types hub. This page is general information, not legal advice.