Building an ADU in Alamo? Your ADU contractor deals with the county, not a city hall.

Unincorporated permitting sounds intimidating and mostly isn’t — it just means one county department wears every hat. Current planning staff issue the ADU permit, building inspection reviews the construction drawings, and both live at the county’s Application and Permit Center in Martinez, with the ePermits portal handling submittals and fee payments online. The Alamo Municipal Advisory Council, whatever you may hear, has no permit authority.

The sequence is planning approval first, then building permit, and the review is ministerial — objective standards, no hearings, no design commission, and a 60-day state clock once the application is complete. The county also runs a preapproved-plan program for ADU vendors, and state law now gives owners of unpermitted units built before 2020 a ministerial legalization path, which the county has codified.

One thing county review is genuinely picky about is the site plan. The checklist wants setbacks, water and sewer lines for detached units, the parking space drawn to size, and — this one surprises people — the location, species, and size of every tree over 20 inches around within 50 feet of the construction area. In a community shaded by protected oaks, that survey is worth doing carefully the first time.

The county caps detached ADUs at 1,000 square feet on small lots — but at 1,200 square feet on lots of 12,000 square feet or more, and Alamo’s R-20 and R-40 zoning starts at 20,000. In practice, nearly every conventional Alamo lot qualifies for the full 1,200. On top of that, a detached ADU can carry an attached garage — 600 square feet on most Alamo lots, 800 on five-plus acres — that doesn’t count toward the unit’s size at all. Attached ADUs follow the same caps plus a half-the-main-house rule that can’t push a one-bedroom below 850 square feet.

Setbacks are 4 feet at the sides and rear for new construction, with the zone’s normal front setback. Conversions of existing structures need no setbacks and can add 150 square feet for entry and egress, and the state-exempt 800-square-foot detached unit — free of the county’s location and development standards entirely — can stack with a conversion ADU and a JADU on the same lot.

And the design rules? The 2025 rewrite removed the architectural-compatibility requirement that used to apply. What remains is objective and minimal: a permanent foundation, complete independent living facilities, and an exterior entrance on the side or rear of the building or otherwise out of view from the street. One parking space is required for a new ADU — tandem in the driveway satisfies it — with no replacement parking when a garage or even an uncovered space gives way to the unit. Owner-occupancy is not required; rentals must run 30 days or longer.

The single biggest feasibility question on a semi-rural Alamo lot is what happens to the wastewater. On sewered lots, Central San’s ADU capacity fee is $3.63 per square foot, charged on at most 1,200 square feet — under $3,000 for an 800-square-foot unit, against $8,539 for a new house. Its piping rule: a detached ADU connects to the house’s lateral downstream of the main cleanout, never back through the house plumbing, and Central San reviews plans on paper only, so we start that submittal early.

On septic, the county’s ordinance is explicit: the system must be approved by the county health officer before an ADU can be established, through Environmental Health’s land-use program. Whether the existing system can absorb an ADU is a per-site engineering question — and for lots where the answer is no, Central San’s septic-to-sewer program can finance up to $65,000 of a public-sewer connection. This fork in the road is the first thing we scope on any Alamo project.

The rest of the fee sheet: the county’s ADU planning permit runs on a deposit-plus-time-and-materials basis (a $2,000 minimum deposit on the county’s published schedule — confirm current numbers when you apply), building-permit fees are valuation-based, and San Ramon Valley Unified charges $5.17 per square foot on units over 500 square feet, paid directly to the district. State law bars impact fees below 750 square feet. On water, EBMUD’s capacity charge — roughly $19,500 for a standard residential meter — applies per new meter, so an ADU served through the existing house meter avoids it entirely.

Alamo’s lots are the reason to build here and the reason to plan carefully. Creekside parcels — and San Ramon Creek and its tributaries thread through town — carry county structure setbacks of 30 to 50 feet from the top of the bank depending on its height, verified before a building permit issues. Slopes over 5 percent can trigger a geotechnical report. Neither is a stopper; both belong in the site plan before design, not after.

Fire review comes from the San Ramon Valley Fire Protection District, and the 2025 state fire-hazard maps put parts of the SRVFPD area — including Alamo’s hillside edges toward Las Trampas and the Diablo foothills — in elevated severity zones. Check your parcel on the district’s map viewer early; elevated zones bring ignition-resistant construction and defensible-space requirements, and the district signs off before final inspection.

Finally, Alamo is HOA country, and the question comes up constantly: state law voids CC&R provisions that effectively prohibit or unreasonably restrict ADUs, and bars HOAs from attaching fees or financial requirements to them. Reasonable, cost-neutral design input can survive; a veto cannot.