Detached ADU
Detached ADU: the most flexible and most expensive option.
A detached ADU is a standalone secondary structure built from the ground up in your backyard or on your lot. It's the most common ADU type for homeowners building from scratch — and the most expensive — but it delivers full design flexibility, full privacy, and the strongest rental and resale story of any ADU type.
What it is
A fully independent dwelling unit, structurally separate from the primary home. It has its own foundation, framing, roof, kitchen, bathroom, and utility connections. Most California cities allow detached ADUs up to 1,200 sq ft on single-family lots.
- Cost range
- $200K–$400K+ (CA 2026, DQB/Kellow); $350K–$650K+ in the Bay Area (Barcci 2026)
- Per sq ft
- $350–$500+ per sq ft in California
- Size cap
- Up to 1,200 sq ft in most California cities; 850 sq ft for studios and 1-bedrooms by default under state law
- Timeline
- 18–32 weeks construction on a standard lot; 28–40+ weeks on hillside or complex sites (ACI 2026)
Pros
- Full design flexibility — orientation, layout, ceiling height, all designable from scratch
- Maximum privacy for both you and the tenant — no shared walls or entry
- Highest rental income potential — CA detached 1BRs rent $1,800–$5,500/mo depending on metro
- Strongest resale value uplift — 50–100% of build cost per DQB 2026
- No owner-occupancy requirement in California since 2020
Cons
- Highest absolute cost of any ADU type
- Longest construction timeline — 18–32 weeks before site/permits
- Requires lot space meeting setback requirements (4 ft minimum side/rear under CA state law)
- Most exposure to site conditions — slope, soil, and utility distance all add cost
Rules and requirements
| Maximum size | 1,200 sq ft (most CA cities) |
| Minimum setbacks | 4 ft side and rear under CA state law for units ≤16 ft tall |
| Owner-occupancy | Not required in CA since 2020 (AB 881) |
| Parking | Waived within ½ mile of transit per CA state law |
| Impact fees | Waived under 750 sq ft; proportional above 750 sq ft |
Best for
- Long-term rental income or housing for an aging family member who wants independence
- Lots with adequate yard space after setbacks
- Homeowners who value design control and willing to commit to a longer build
- Premium California rental markets where 1BR detached units rent $3,000+/mo
Get sharper numbers for your specific project
Run the calculator with your inputs, or get matched with builders who can quote your lot directly.
FAQ
- How big can a detached ADU be?
- California state law allows detached ADUs up to 850 sq ft for studios and 1-bedrooms and 1,000 sq ft for 2-or-more-bedroom units. Many cities allow up to 1,200 sq ft on lots with adequate coverage. The HCD ADU Handbook (March 2026) is the authoritative reference.
- Do I need to provide parking for a detached ADU?
- California state law eliminates parking requirements for ADUs within half a mile of public transit, in historic districts, or where on-street parking permits aren't required. In practice, most California metropolitan detached ADUs qualify for the exemption.
- How close to the property line can I build?
- 4 feet minimum from side and rear property lines under California state law for ADUs 16 feet tall or shorter. Taller ADUs require larger setbacks. Front-yard ADUs may have different rules.
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