ADU type
Attached ADU
An attached ADU shares one or more walls with the existing house. You save on a second foundation and one shell wall, but you inherit the main house's structural system — which decides whether the project pencils.
What it is
- New construction physically attached to the primary residence — shares at least one wall.
- Capped at 50% of the existing house size or 1,200 sqft (whichever is smaller) by state law.
- Must have its own exterior entrance — internal pass-through doors are optional.
- Typically shares foundation footprint or extends it, which can require underpinning if the existing footing is shallow.
- Can share HVAC, water heater, and electrical service if capacity allows — saves $15K–$30K vs. detached.
Los Angeles · 2026
$280K – $460K all-in
$380 – $560/sqft
Savings vs. detached come from shared foundation, wall, and utilities — typically $40K–$80K.
Bay Area · 2026
$360K – $620K all-in
$490 – $760/sqft
Watch for underpinning and main-house structural retrofits — they can erase the savings.
When it fits
- You have a side yard but not enough rear yard for a true detached structure.
- The existing house's foundation, electrical, and HVAC have enough headroom to share.
- You want a faster, cheaper build than detached but more space than a garage conversion.
- You plan to rent to family — internal connecting doors give optional flexibility.
When it doesn't
- Existing house is on a raised foundation with crawl space — matching it is expensive.
- Title 24 mechanical separation requirements make shared HVAC impractical (common in older bungalows).
- You're hitting FAR limits — attached counts toward house square footage in some jurisdictions.
Permit pathway
Los Angeles
Same ministerial ADU pathway. Structural calcs reviewed against the existing house's plans — expect a request for the original framing drawings or an existing-conditions survey.
Bay Area
Ministerial. Seismic retrofit of the existing wall may be required where the new opening interrupts shear continuity — adds 2–4 weeks of engineering.
Realistic timeline
Slightly faster than detached: 10–16 months door-to-door, mainly because the foundation and one wall are already there.
FAQ
- Does an attached ADU need a separate address?
- Yes — the post office and county assessor treat it as a separate dwelling. You'll get a Unit A / Unit B addressing or a new street number assigned at permit.
- Can I cut an interior door between the house and the ADU later?
- Yes, but the opening must be rated and protected with a self-closing fire door. The ADU's exterior entrance is still required — you can't downgrade an ADU to a 'room addition' once permitted.
- Do I need fire-rated walls between the house and the ADU?
- Yes — typically a 1-hour fire-rated assembly on the shared wall and ceiling. This is a code requirement, not negotiable.
- Is an attached ADU cheaper than a home addition of the same size?
- Usually yes, because ADU regulations are ministerial and skip discretionary design review. An addition often triggers neighbor notice and a design review hearing that adds 3–6 months.