ADU Builder in Belmont, CA
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ADUs in Belmont
Belmont offers a pre-approved ADU program: designer plans are reviewed and posted to a gallery, and site-specific applications using them are reviewed within 30 days. State ADU law guarantees an ADU of up to 800 sq ft with 4-foot side and rear setbacks and a 16-foot height on a single-family lot.
In December 2025, the state housing department found parts of Belmont’s 2020 ADU ordinance out of line with state law, so we confirm current local standards with the Permit Center for each project.
ADU Construction
Permits and Fire
Fire and sprinklers: San Mateo Consolidated Fire serves Belmont. Its standard requires a sprinkler retrofit when work covers more than 50% of the existing floor area, and detached ADUs are exempt unless the main house is sprinklered.
Hillside lots: the San Juan Hills and Western Hills Area Plans govern hillside lots, which can involve steep slopes, geologic hazards, access improvements and lot-legality questions. Grading over 50 cubic yards, more than 2 feet of cut or fill, or over 2,000 sq ft disturbed triggers grading and geologic review.
City Hall, One Twin Pines Lane, Suite 110, Belmont, CA 94002 · (650) 595-7422, pe**********@*****nt.gov
Walk-in hours Monday, Wednesday and Thursday, 8:30 to 11:30 a.m., with Building, Planning and Engineering staff; appointments at other times. Permits and inspections run through eTRAKiT, with morning (9 to noon) and afternoon (1 to 4) inspection windows.
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Other Belmont services: Bathrooms · Kitchens · Additions · New Homes · Belmont overview
Same service nearby: San Carlos · San Mateo · Hillsborough
See how to build an ADU.
Belmont ADU Builder FAQ
Does Belmont have pre-approved ADU plans?
Yes. Site-specific applications using them are reviewed within 30 days.
What size ADU does state law guarantee?
Up to 800 sq ft with 4-foot side and rear setbacks and 16 feet of height.
Does a detached ADU need sprinklers?
Not unless the main house is sprinklered.
Can I build an ADU on a hillside lot?
Yes, with grading and geologic review where triggered.