ADU Builder in San Martin, CA
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County ADU Rules on Rural Lots
San Martin ADUs follow Santa Clara County Zoning Ordinance ยง4.10.015. Current standards:
- Detached: up to 1,200 sq ft, and an attached junior ADU can add 500 sq ft, for 1,700 sq ft total.
- Attached: up to 800 sq ft if the main house is 1,600 sq ft or less; up to 50% of the house for homes of 1,601 to 2,400 sq ft; up to 1,200 sq ft for homes over 2,400 sq ft.
- Height: 16 feet, or the district height if the ADU meets the house setbacks; detached ADUs over 16 feet need a hip or gable roof.
- Setbacks: 4 feet side and rear; front setback the same as the house.
- Parking: one space with state exemptions; none for a JADU. A JADU requires owner occupancy.
- Ministerial review within 60 days.
Changes are coming: the state flagged parts of the County ordinance in June 2025, and amendments are scheduled for SMPAC in November 2026, the Planning Commission in December 2026 and the Board in early 2027. We track the changes for your project.
ADU Construction
Septic, Wells and Site Approval
Before an ADU application can be submitted, the main house must have building site approval. Because most San Martin parcels are on septic and wells, the ADU design starts with Environmental Health requirements.
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.
County Pre-Approved ADU Plans
Santa Clara County offers free pre-approved ADU plans: an 800 sq ft two-bedroom, one-bath unit and a 1,200 sq ft three-bedroom, two-bath unit, each in Traditional or Contemporary style. Parcels in wildland-urban interface, flood, geologic-hazard or historic areas are excluded, and plan review takes about four weeks.
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.
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Other San Martin services: Bathrooms · Kitchens · Additions · New Homes · San Martin overview
Same service nearby: Morgan Hill · Gilroy
San Martin ADU FAQ
How big can an ADU be in San Martin?
Up to 1,200 sq ft detached, plus a 500 sq ft attached junior ADU, under current County rules.
Does a San Martin ADU need septic review?
Yes. County Environmental Health reviews septic for ADUs.
Are County ADU rules changing?
Yes. Amendments are scheduled for hearings from November 2026 into early 2027.
Does the County offer pre-approved plans?
Yes, free 800 and 1,200 sq ft plans in Traditional or Contemporary styles, with some parcel exclusions.