ADU Builder in Alamo, CA
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Alamo ADUs Follow Contra Costa County Rules
Because Alamo is unincorporated, ADUs here follow the County Ordinance Code, Chapter 82-24, rewritten in May 2025 (Ordinance 2025-07). Approval is ministerial, with no discretionary review and no public hearing. The ADU permit application goes to the County before the building permit application.
- Detached ADU: up to 1,000 sq ft, or up to 1,200 sq ft on lots of 12,000 sq ft or more, which covers many Alamo parcels.
- Attached ADU: the same caps and no more than 50% of the main home, but never limited below 850 sq ft for one bedroom or 1,000 sq ft for two or more.
- State-law ADU: a detached unit up to 800 sq ft with 4-foot side and rear setbacks, plus a conversion ADU and a junior ADU.
- Setbacks: 4 feet side and rear; none for conversions. Front yards follow the zoning district.
- Parking: one space, which can be tandem or in a setback, with exemptions for transit, conversions, and ADUs applied for with a new home.
- Garages converted to ADUs do not need replacement parking. Rentals must be at least 30 days.
ADU Construction
County Pre-Approved ADU Plans
Contra Costa County runs a pre-approved ADU plan program for vendors’ plans. Each site still needs its own site plan, foundation plan and permits, which we prepare.
Department of Conservation and Development, Application and Permit Center
30 Muir Road, Martinez, CA 94553 · (925) 655-2700
Monday to Thursday 7:30 a.m. to 5 p.m., Friday 7:30 a.m. to 4 p.m. Applications are filed online through the County ePermits Center; plan review is fully electronic.
Alamo Site Factors
- Septic: some Alamo parcels are still on septic. Any improvement on a septic property needs County Environmental Health review, and health officer approval is verified before final.
- Water: EBMUD serves Alamo; some hillside parcels had to be annexed for service.
- Creeks: along unimproved channels, buildings must stay back 30 to 50 feet from the top of bank, depending on bank height.
- Protected trees: see the County tree ordinance notes on our Alamo additions page.
County inspections: request through ePermits or the automated line at (925) 646-4108. Phone requests for same-day inspections are accepted until 5:30 a.m., Monday through Friday. The permit and stamped plans must be on site, with an adult present.
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Other Alamo services: Bathrooms · Kitchens · Additions · New Homes · Alamo overview
Same service nearby: Danville · Walnut Creek · Lafayette
Alamo ADU FAQ
Who approves ADUs in Alamo?
Contra Costa County, because Alamo is unincorporated. ADU review is ministerial with no public hearing.
How big can an ADU be in Alamo?
Up to 1,000 sq ft, or up to 1,200 sq ft on lots of 12,000 sq ft or more.
Does the Alamo MAC approve ADUs?
No. The MAC is an advisory body; the County makes decisions.
Can I build an ADU on a septic lot in Alamo?
Often, with County Environmental Health review of the septic system.