A homeowner on Danville Boulevard hears that Contra Costa County rewrote its accessory dwelling unit rules last year and assumes the hard part is over. The lot is big enough. The design no longer has to match the house. All that's left is picking a contractor and choosing paint colors.
Then the permit conversation starts, and the real decision surfaces: what happens to the wastewater. That single question, sewer or septic, and if sewer, whether the new unit gets its own water meter, moves the project's cost by tens of thousands of dollars before a single stud goes up. The ordinance rewrite is real and it matters. It is also not the thing deciding whether an Alamo ADU pencils out.
The rewrite everyone's talking about
Contra Costa County rewrote its ADU ordinance (Ordinance 2025-07, County Code Chapter 82-24) in 2025, and the changes were kind to Alamo specifically. The county's large-lot allowance for a full 1,200-square-foot detached unit kicks in at 12,000 square feet. Alamo's R-20 zoning starts at 20,000 square feet, which means nearly every conventionally zoned lot in town already clears that bar. The old requirement that a new unit architecturally match the main house is gone. An attached garage of 600 to 800 square feet doesn't count against the size cap either, so a homeowner can build a full-size ADU with a garage bay attached and still be sized under the 1,200-square-foot ceiling for permitting purposes.
That's the part getting written about. It's also the part that was never really the obstacle. Alamo's lots were already big enough for most owners to build something. The ordinance rewrite removed a design constraint. It didn't touch the cost structure underneath.
The fork under the lawn
Alamo has no city hall. It's unincorporated Contra Costa County, which means ADU permits run through the county's Department of Conservation and Development, either through the online ePermits portal or in person at 30 Muir Road in Martinez. The Alamo Municipal Advisory Council can weigh in on county policy, but it doesn't issue permits and it doesn't decide your fee schedule.
What decides the fee schedule is which utility serves the parcel.
On a sewered lot, Central Contra Costa Sanitary District charges an ADU capacity fee of $3.63 per square foot, capped at 1,200 square feet. That works out to under $3,000 for an 800-square-foot unit, against $8,539 for a full new-house connection. The district's piping rule is specific: a detached ADU connects to the house's existing sewer lateral downstream of the main cleanout and never routes back through the house plumbing, and Central San reviews the plans on paper rather than requiring a site visit before the permit moves forward. That's a fast, cheap path if the house is already on public sewer.
On a septic lot, there's no flat fee. Contra Costa County's Environmental Health division has to approve the existing system before an ADU can be established, and whether that system can absorb the added load is a per-site engineering question, not a formula. If the answer comes back no, the owner is looking at either an expensive engineered septic upgrade or a full septic-to-sewer conversion, which is a different project with a different price tag entirely.
Three numbers that actually decide the math
| What triggers it | What it costs | When it applies |
|---|---|---|
| Central San ADU sewer capacity fee | $3.63 per square foot, capped at 1,200 sq ft | Lot is already on public sewer |
| New EBMUD water meter | Roughly $19,500 | Only if the ADU gets its own meter instead of sharing the house's existing one |
| Septic capacity review | No set fee; per-site engineering evaluation by County Environmental Health | Lot is on septic, required before the ADU can be established |
| Central San Septic-to-Sewer financing | Up to $65,000, repaid over 15 years through the county property tax roll | Existing septic system can't absorb the added load |
| SRVUSD school facilities fee | $5.17 per square foot on units over 500 sq ft | Any ADU over 500 square feet, on top of everything else |
None of these numbers show up in a zoning summary. All of them show up on the invoice.
The meter decision nobody flags up front
EBMUD's system capacity charge runs roughly $19,500, but it only applies when a new water meter gets added for the ADU. A unit designed to run off the house's existing meter avoids the charge entirely. That's a design decision, not a permitting formality, and it's the kind of detail that separates a $15,000 backyard project from one that clears $30,000 before construction starts. Anyone budgeting an ADU off a contractor's rough estimate should ask directly whether the plan includes a separate meter, because that single line item is larger than most of the county's own fees combined.
The septic path rewards getting an answer early
For homes still on septic, the sequence matters. County Environmental Health has to sign off on the system's capacity before the ADU can move forward, and that determination is specific to the parcel, the age of the system, and how much additional load a second dwelling would add. If the system passes, the path looks similar to a sewered lot, just without Central San's fee schedule to plan around.
If it doesn't pass, Central San's Septic to Sewer financing program becomes the relevant number. It finances up to $65,000 toward the cost of abandoning the septic system and connecting to public sewer, repaid over 15 years through the county property tax roll rather than as an upfront cash outlay. Separately, the district's own connection and permit fees for a standard conversion currently run in the $9,200 to $10,200 range per residential unit, before the cost of the side sewer construction and septic tank abandonment. Those figures move the ADU conversation into a different category of project, and they're worth pricing out before assuming the backyard is a quick build.
The fee that ignores the size line everyone quotes
State law generally waives local impact fees for ADUs under 750 square feet, which is the number most owners hear and assume covers everything. It doesn't cover the San Ramon Valley Unified School District's facilities fee, which applies at $5.17 per square foot starting at just 500 square feet. A homeowner who designs a small unit specifically to dodge impact fees can still end up owing a school fee on it, because that fee runs on its own threshold rather than the one everyone's repeating.
Who actually has authority here
Because Alamo is unincorporated, review is ministerial rather than discretionary. Meet the standards and the county approves. A state law that took effect January 1, 2026 requires local agencies to determine within 15 days whether an ADU application is complete, and applicants who don't get an answer in that window can treat the application as complete by default. That's a meaningful change from the slower back-and-forth reviews owners describe from a few years ago.
CC&Rs don't get a vote either. State law voids homeowners association provisions that prohibit or unreasonably restrict ADUs on single-family lots, and it specifically bars HOAs from imposing fees or financial conditions that state law doesn't otherwise allow. An Alamo owner in a community with an association still answers to the county's ordinance first.
A short FAQ
Does the ADU have to match the main house? No. The county's 2025 ordinance dropped that requirement.
Do I have to live on the property to build one? For a standard ADU, no. Owner-occupancy requirements were removed under state law. A junior ADU only triggers an occupancy requirement if it shares a bathroom with the main house.
Can my HOA block it? No. State law preempts CC&R provisions that would prohibit or unreasonably restrict a single-family ADU.
What if my lot is on septic and the system can't take the extra load? The county's Environmental Health division makes that determination on a per-site basis, and Central San's Septic to Sewer financing program can cover up to $65,000 of the conversion cost if a switch to public sewer becomes necessary.
The number that actually matters
An ADU in Alamo isn't priced by the ordinance. It's priced by what's already running under the lot, sewer or septic, one meter or two, and by a school fee that starts counting well before the size threshold most people assume protects them. The zoning news is the part that's easy to write up. The utility question is the part that decides whether the project makes financial sense, and it's worth answering before a design gets drawn, not after.
If you're weighing whether your Alamo property has room for an ADU, or you're evaluating a listing where one's already built, the Mary Bonham Team can walk through what a given parcel's utility situation means for value and next steps. Let's Connect.
