Contra Costa County, CA Building Setbacks & Zoning: Lot Coverage Limits (2026)
Key Facts
- Standard R-6 lot coverage cap
- None; governed by setbacks/yards instead
- SB9 coverage, lot under 3,000 sf
- 67% maximum
- SB9 coverage, 3,000-6,000 sf lot
- 50% maximum
- SB9 coverage, 6,000-12,000 sf lot
- 40% maximum
- SB9 coverage, 12,000+ sf lot
- 30% maximum
- Governing ordinance
- Ord. No. 2022-14, § II, 3-29-22
Summary
Unincorporated Contra Costa County does not cap ordinary single-family lot coverage by percentage; the county's lot, yard, and height rules govern instead. But when a lot adds a second unit under the county's two-unit 'urban housing development' ordinance, Section 88-36.012(d) caps the combined structures at 30 to 67 percent of the lot, sliding down as the lot gets bigger.
(d)Lot Coverage.(1)Except as otherwise provided in this subsection (d), the structures included in an urban housing development may not cover more than:(A)67 percent of the lot area on a lot of less than 3,000 square feet.(B)50 percent of the lot area on a lot of 3,000 square feet or more but less than 6,000 square feet.(C)40 percent of the lot area on a lot of 6,000 square feet or more but less than 12,000 square feet.(D)30 percent of the lot area on a lot of 12,000 square feet or more.(2)The lot coverage limitations of this subsection may be exceeded to allow construction of one new residential unit on a lot with an existing residential unit, if the new residential unit does not exceed:(A)800 square feet in size on a lot of less than 6,000 square feet.(B)1,000 square feet in size on a lot of 6,000 square feet or more but less than 12,000 square feet.(C)1,200 square feet in size on a lot of 12,000 square feet or more.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Ordinance Code: Supplement 103).
Full Breakdown
Contra Costa County's base single-family zoning chapters, such as Chapter 84-4 for the R-6 district, set minimum lot area, width, and depth, a maximum building height, and front, side, and rear yard setbacks, but none of them cap total lot coverage by percentage. The percentage cap that does exist in the code applies specifically to a two-unit residential development authorized under Chapter 88-36, the county's implementation of the state's SB 9 duplex law.
Section 88-36.012(d)(1) sets a sliding scale: the structures in an urban housing development may not cover more than sixty-seven percent of the lot area on a lot under 3,000 square feet, fifty percent on a lot of 3,000 up to 6,000 square feet, forty percent on a lot of 6,000 up to 12,000 square feet, or thirty percent on a lot of 12,000 square feet or more. Subsection (d)(2) allows that cap to be exceeded to add one new unit to a lot that already has an existing unit, so long as the new unit stays within a separate size limit: 800 square feet on a lot under 6,000 square feet, 1,000 square feet on a lot of 6,000 to 12,000 square feet, or 1,200 square feet on a larger lot.
These developments still must meet ordinary yard and height rules for the underlying zone, except that Section 88-36.012(e) requires only a four-foot side and rear setback, rather than the zone's standard setback, for a new unit that is not built in the footprint of an existing building.
Violations & Fines
A structure that exceeds the Section 88-36.012(d) lot-coverage tier for its lot size does not qualify as a ministerial two-unit urban housing development under Section 88-36.006(a), which conditions ministerial, no-hearing approval on meeting the location standards in Section 88-36.010 and the development standards in Section 88-36.012, coverage included. An application that fails that test isn't automatically denied outright but drops out of this streamlined ordinance and must instead be pursued, if at all, as a conventional discretionary land use or variance permit under the county's general zoning procedures.
Frequently Asked Questions
Is there a maximum lot coverage percentage for a house in unincorporated Contra Costa County?
What is the lot coverage limit for adding a second unit under Contra Costa's SB9 ordinance?
What setback applies to a new unit built under the two-unit ordinance?
Sources & Official References
Other rules in Contra Costa County
California rules heatmap·Compare Contra Costa County to another location·View the California building setbacks & zoning overview
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