ADU type
Basement ADU
A basement ADU converts unfinished or under-utilized below-grade space into a permitted unit. The shell exists, but egress windows, headroom, and waterproofing decide whether the cost lands closer to a garage conversion or closer to new construction.
What it is
- Conversion of below-grade or partially below-grade space into a permitted ADU.
- Most common in older Bay Area neighborhoods (Inner Richmond, Outer Sunset, Berkeley flats, parts of Oakland) and LA hillside homes with daylight basements.
- Requires code-compliant egress: a window of at least 5.7 sqft openable area at sill height ≤44" — often the most expensive part of the project.
- Headroom must be 7 ft minimum in habitable rooms; many older basements have 6'-6" or less and need a slab cut-and-lower.
- Moisture management (perimeter drainage, sump pump, vapor barrier) is non-negotiable.
Los Angeles · 2026
$220K – $410K all-in
$380 – $560/sqft
LA basement ADUs are rare — most LA homes are slab-on-grade. When they exist, egress and waterproofing dominate the budget.
Bay Area · 2026
$280K – $520K all-in
$440 – $720/sqft
Common in San Francisco and Berkeley. Soft-story compliance and SFPUC sewer-lateral compliance often add $25K–$60K.
When it fits
- Basement has 7+ ft of headroom and at least one wall exposed to grade.
- Existing exterior stairwell or daylight window well is already there or easy to add.
- House is in a Bay Area neighborhood where basement units are normal and rentable.
- You want a separate unit without giving up yard or street parking.
When it doesn't
- Headroom is under 6'-6" — slab lowering can add $80K–$150K.
- Lot has a high water table — moisture control becomes the whole project.
- House is on a true crawl space, not a basement — converting a crawl space is effectively new construction below the house.
Permit pathway
Los Angeles
Ministerial under state ADU law. LADBS requires soils information for any below-grade construction. Hillside Ordinance overlay common — adds drainage and grading review.
Bay Area
Ministerial. SF requires soft-story screening for buildings of certain ages — basement work is often the trigger for the retrofit. Expect 2–4 weeks of additional structural engineering.
Realistic timeline
Design + permit: 12–20 weeks (soils, structural, waterproofing engineering). Construction: 5–8 months. Plan on 11–16 months door-to-door.
FAQ
- Can I rent out my finished basement without permits?
- Legally, no — unpermitted units can't be advertised, are uninsurable, and create disclosure problems at sale. The Costa-Hawkins protections and rent-control rules also generally don't apply to unpermitted units, which puts the landlord at risk.
- How much does egress window installation cost?
- $8K–$22K per opening in LA, $12K–$35K in the Bay Area — higher when the work requires shoring an exterior wall or excavating a window well below sidewalk grade.
- Do I need a fire-rated ceiling between the basement ADU and the house above?
- Yes — typically a 1-hour rated assembly. Existing plaster-and-lath ceilings sometimes qualify with minor upgrades; drywall ceilings usually need a second layer of 5/8" Type X.
- What about radon in basement ADUs?
- Radon is uncommon in most of California but does appear in pockets of the Bay Area. A short-term test is cheap ($30 kit) and worth running before the slab is sealed — adding mitigation later costs 3–5× more.