Permitting · 2 min read · April 15, 2026 · 392 words
ADU vs SB9 lot split: which path actually fits your lot
Two state laws now let LA homeowners add density. SB9 splits the lot and creates a separately sellable parcel; ADU law adds a unit. The decision turns on three numbers — and one local opt-out.
Key takeaways
- Lot size. SB9 requires the resulting parcels to be at least 1,200 sq ft each. Below 2,400 sq ft total, SB9 is off the table.
- Slope. SB9 explicitly excludes lots with average slope greater than 25%. Most LA hillside lots fail this test — see hillside ADUs for the geo report context.
- Hazard overlays. Very High Fire Hazard Severity Zones, Alquist-Priolo Earthquake Fault Zones, and floodways are SB9-ineligible by statute.
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SB9 (the California HOME Act) and ADU law solve different problems. ADU law lets you add up to two units to an existing lot. SB9 lets you split a single-family lot into two parcels and build up to two units on each. They're stackable in theory, but LA has restrictive overlays that often force a choice — and the LA ADU permitting rules govern the ADU lane.
The three numbers that decide
- Lot size. SB9 requires the resulting parcels to be at least 1,200 sq ft each. Below 2,400 sq ft total, SB9 is off the table.
- Slope. SB9 explicitly excludes lots with average slope greater than 25%. Most LA hillside lots fail this test — see hillside ADUs for the geo report context.
- Hazard overlays. Very High Fire Hazard Severity Zones, Alquist-Priolo Earthquake Fault Zones, and floodways are SB9-ineligible by statute.
What SB9 actually unlocks
SB9 splits a lot 50/50 (or close to it), produces two separately sellable parcels, and allows up to two units on each parcel — for a theoretical maximum of four units. In practice, LA's enforcement of the owner-occupancy affidavit and the requirement that the splitter live on-site for three years dampens speculative use.
What ADU law unlocks
An ADU stays on one parcel. Depending on the property and current law, a detached ADU and a JADU may be possible. Government Code § 66317 supplies a ministerial completed-application decision process, but it is not an all-in permit or construction duration. Compare the legal paths and property-specific documents rather than assuming one is automatically faster or cheaper. The permit-path guide explains the clock boundary.
Decision matrix
- Want to sell off the new unit separately → SB9 (with all its caveats).
- Want rental income, fastest path, lowest soft costs → ADU. The rental yield guide sizes the income side.
- Lot under 2,400 sq ft, hillside, or in a hazard overlay → ADU.
- Lot over 4,000 sq ft on a flat parcel outside hazard zones → SB9 worth modeling.
Sources
- SB9 Fact Sheet · California HCD
- California SB9 (2021) Bill Text · California Legislature
- Alquist-Priolo Earthquake Fault Zones · California Geological Survey
- LADBS Permits & Bulletins · LA Department of Building and Safety
Next chapter · 01 of 03
Permitting · 4 min read
Los Angeles ADU permitting rules: what applies now
If ADU is the lane, review the current City of LA application rules and their exceptions.
Current California ADU rules for City of Los Angeles projects: application review, setbacks, height, occupancy, fees and site-specific documents.
FAQ · Permitting
Questions about this guide
What is SB-9 and does it apply to me?
SB 9 creates a state framework for qualifying housing development and urban lot splits, but eligibility and the practical development path are property-specific. Review the current statute, local implementation materials, tenancy and demolition restrictions, site constraints and title questions before relying on a unit count.
Continue your read · the editorial path
We chained these chapters in the order LA homeowners actually need them. Each one picks up where the last one left a question open.
02 / 03
Finance · 2 min
Will your ADU trigger a property tax reassessment?
Under Prop 13 and BOE Rule 463, only the new construction value is reassessed — not the entire property. A practical walkthrough of how the LA County Assessor calculates the bump.
Read chapter →03 / 03
Cost · 4 min
The anatomy of an LA ADU build, line by line
How to organize an LA detached ADU budget — soft costs, site work, construction, contingency, and the unknowns to resolve before comparing totals.
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