Home Additions in San Martin, CA
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Additions Start With Environmental Health
In San Martin, County Environmental Health reviews septic for both major and minor additions. An addition with new bedrooms can require a larger drainfield, and a structure tied to an existing system needs a pumper report and a 30-minute water test. We schedule this review at the start, not the end.
Septic and wells: most of San Martin has no public water or sewer. County Environmental Health reviews onsite wastewater (septic) systems for new homes, ADUs and both major and minor additions. Drainfields are sized by soil percolation rate and number of bedrooms, and connecting a structure to an existing system requires a pumper report and a 30-minute water test. The San Martin plan prohibits new septic systems in the highly permeable soils along Llagas Creek. Most homes use private or shared wells; the San Martin County Water District serves part of the area east of Monterey Road, and California American Water acquired West San Martin Water Works in 2025.
Addition Work
Rural Zoning and Design
- Rural Residential density uses a 5 to 20 acre variable slope density formula, and cluster lots must be at least 1 acre.
- The 1995 San Martin Integrated Design Plan guides design.
- Agriculture is encouraged, and new development must be compatible with farming.
- Highway 101 through San Martin is treated as a scenic highway.
- Projects needing Planning Commission or Board approval go to the San Martin Planning Advisory Committee.
San Martin Planning Advisory Committee (SMPAC) advises the County Planning Commission and Board of Supervisors, reviewing San Martin applications that need Planning Commission or Board approval and code or General Plan changes. It meets monthly, on the Wednesday before the Planning Commission’s first-Thursday meeting.
County Permits
Department of Planning and Development
70 W. Hedding Street, East Wing, 7th Floor, San Jose, CA 95110 · (408) 299-5700
Monday to Thursday 8 a.m. to noon and 1 to 5 p.m.; Fridays limited building services for active permits. Applications go through the County Public Permit Portal. Environmental Health, Roads and Fire Marshal staff are at the permit center on Tuesdays and Thursdays.
County inspections: book through the VuSpex virtual inspection app, the Public Permit Portal or (408) 299-5700; requests before 3 p.m. on a workday get a next-day inspection. Fire Marshal inspections: (408) 341-4420. Request a final inspection on every related permit to close out the project.
Fire: Santa Clara County Fire Department took over from the South Santa Clara County Fire District on July 1, 2025; fire plan review for unincorporated areas goes through the County.
Explore More
Other San Martin services: ADUs · Bathrooms · Kitchens · New Homes · San Martin overview
Same service nearby: Morgan Hill · Gilroy
See home additions.
San Martin Home Additions FAQ
Does a small addition need septic review in San Martin?
Yes. Environmental Health reviews both major and minor additions.
What is SMPAC?
The San Martin Planning Advisory Committee, which advises the County on San Martin applications that need Planning Commission or Board approval.
Who provides fire service?
Santa Clara County Fire Department, since July 1, 2025.
Who issues the building permit?
Santa Clara County Department of Planning and Development.