Attached ADU
Attached ADU: a separate unit that shares a wall with the main home.
An attached ADU is built as an extension of the primary home — sharing at least one wall — with its own kitchen, bathroom, and separate exterior entrance. It's typically cheaper than a detached ADU because foundation and roof tie into the existing structure, and quicker to build because the shared envelope is already there.
What it is
A dwelling unit attached to the primary home, structurally tied in but functionally independent. Has its own kitchen, bath, and exterior entry. Common configurations: addition off the side or rear of the house, or above a garage.
- Cost range
- $150K–$300K (CA 2026, DQB/Kellow); $280K–$480K Bay Area (Barcci 2026)
- Per sq ft
- $300–$450 per sq ft (DQB 2026)
- Size cap
- Up to 1,200 sq ft (some CA cities); 50% of primary home's living area is a common state-law cap
- Timeline
- 16–28 weeks construction (ACI 2026)
Pros
- Lower cost than detached — shared foundation, walls, and utilities reduce hard costs
- Faster construction than detached — typically 4–8 weeks shorter
- Easier to extend utilities from main house (especially plumbing)
- Works on narrower lots that can't fit a detached structure
Cons
- Less privacy than detached — shared wall, closer proximity to main home
- Construction is more disruptive to the primary home (dust, noise, structural work)
- Tying into existing structure can surface hidden conditions (foundation, framing, code upgrades)
- Sound transmission across shared walls is a real design issue — budget for STC-rated assemblies
Rules and requirements
| Maximum size | 50% of primary home or 1,200 sq ft, whichever is less (varies by city) |
| Owner-occupancy | Not required in CA since 2020 |
| Parking | Same exemptions as detached ADUs apply |
| Impact fees | Waived under 750 sq ft |
Best for
- Lots that can't accommodate a detached structure after setbacks
- Homeowners who want lower absolute cost than detached but still want a fully separate unit
- Multi-generational situations where proximity is a feature, not a bug
- Properties where utility extension to a detached location would be very expensive
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FAQ
- Is an attached ADU the same as an addition?
- No. An addition extends the primary home's living space and shares the main entry — it's not a separate dwelling unit. An attached ADU has its own kitchen, bath, and exterior entrance and is legally a separate residence that can be rented.
- What's the maximum size for an attached ADU?
- California state law commonly caps attached ADUs at 50% of the primary home's living area, with some cities allowing up to 1,200 sq ft regardless. Confirm with your city's planning department for the exact local rule.
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