Two homes on quarter-acre-plus lots in Alamo, priced within a few hundred thousand dollars of each other, can face a $20,000 gap in what it costs to add a primary suite, a pool house, or an ADU. The difference has nothing to do with the contractor, the finishes, or the square footage on the plan. It comes down to a single fact that never shows up in listing photos or square footage totals: whether the property sits on public sewer or on a septic system.
Alamo has no city hall. It is unincorporated Contra Costa County, which means every remodel, addition, and accessory dwelling unit runs through the county's Department of Conservation and Development in Martinez rather than a local planning office. That single detail changes how renovation planning has to work here, and the septic question sits at the center of it.
Why the Same Lot Size Doesn't Mean the Same Renovation
Alamo's zoning runs large. Most conventionally zoned lots in town start at 20,000 square feet under the county's R-20 designation, well above the 12,000-square-foot threshold the county set in its 2025 ADU ordinance rewrite for its most generous accessory dwelling allowances. On paper, that means most Alamo homeowners qualify for a detached ADU up to 1,200 square feet, with an attached garage of 600 to 800 square feet that doesn't even count against that limit, and none of the old requirement that the new structure architecturally match the main house.
The zoning is the easy part. What actually determines the project's cost and timeline is the land itself: whether the parcel connects to public sewer or relies on an individual septic system, along with creeks, protected trees, and slope. A septic lot and a sewered lot with identical acreage can require entirely different paths to the same addition.
If a property is on public sewer, Central Contra Costa Sanitary District, known locally as Central San, charges a connection fee of roughly $3.63 per square foot for an ADU, capped at 1,200 square feet. Add a new water meter and EBMUD's system capacity charge lands around $19,500, though a homeowner who routes the ADU through the house's existing meter can avoid that charge entirely.
If the same lot runs on septic, none of those sewer fees apply, but a different process takes over. Contra Costa Environmental Health has to approve the septic design before the county will approve the ADU at all, and that review looks at soil percolation, setbacks from the existing system, and whether the property has enough land left over for a reserve leach field if the current one ever fails. That process has its own timeline, and it can stall a project that a sewered neighbor would clear in weeks.
What the County Actually Reviews
| Renovation cost or step | Sewered lot | Septic lot |
|---|---|---|
| Governing agency | Central San (connection) | Contra Costa Environmental Health |
| ADU connection fee | About $3.63/sq ft, capped at 1,200 sq ft | No sewer fee; septic design and soil review instead |
| New water meter charge | EBMUD capacity charge, about $19,500 | Same charge applies if a new meter is added |
| Additional approval step | Central San permit for exterior sewer work | Percolation test and Environmental Health plan approval before building permit |
| Building plan review type | Standard | Split into two reviews: setback compliance, then sewage disposal method for new fixtures |
Contra Costa's Environmental Health division splits its septic-related review into two distinct steps: one that confirms a new structure sits far enough from the existing septic system and drain field, and a separate review that evaluates how any new plumbing fixtures in a remodel or addition will actually dispose of wastewater. A homeowner adding a bathroom to a septic-lot addition is not just pulling a building permit. They're triggering a sewage disposal review that a sewered neighbor never has to think about.
The 300-Foot Rule Nobody Mentions at the Open House
The detail that catches people off guard has less to do with new construction and more to do with existing homes. If an Alamo property sits on septic and a Central San sewer main runs within 300 feet of it, Environmental Health can require the owner to connect to that public sewer if the septic system fails or if the homeowner undertakes what the district defines as substantial improvements.
That means a homeowner who buys a septic-lot property assuming they can keep the existing system indefinitely may find, partway through planning a significant remodel, that the county expects a sewer connection as a condition of moving forward. It is not a hypothetical fee buried in fine print. It is a live requirement tied to proximity, and it only surfaces once someone starts drawing plans.
Central San runs a Septic to Sewer financing program specifically because this situation is common enough to need a structured path. The program offers low-cost financing for residential owners with septic systems near existing sewer mains across its service area, which includes Alamo along with Danville, Lafayette, Moraga, Orinda, Pleasant Hill, Walnut Creek, and portions of Martinez and San Ramon. The program exists because converting isn't optional for everyone, and the cost of doing it without planning ahead is higher than doing it on a homeowner's own timeline.
What This Means If You're Planning to Sell
For sellers, this is not just a construction planning issue. Central San is direct about the resale consequence: a property on septic is documented to be less marketable at time of sale compared to a similar home already on sewer, in part because septic systems can limit how much a future owner is able to expand the house or build over parts of the lot. On the other side, converting from septic to public sewer is associated with an increase in market value, while a septic system that starts failing can pull value down until it's repaired or the home connects to sewer.
For an Alamo seller weighing whether to invest in a pre-listing renovation, that's a meaningfully different calculation depending on what's underground. An addition on a sewered lot is a straightforward value-add conversation. The same addition on a septic lot needs a conversation about whether the smarter move is the addition itself, a sewer connection first, or both, priced and sequenced correctly so the improvement doesn't stall in county review after a buyer is already in contract.
With Alamo's average home sale price sitting at roughly $2.7 million as of May 2026, a five-figure infrastructure surprise discovered mid-escrow is a real risk to a transaction timeline, not a rounding error.
Before You Write an Offer or Sign a Remodel Contract
- Ask the seller's agent or the county's ePermits portal whether the specific parcel is on septic or public sewer. This is not always obvious from the address or lot size alone.
- If the home is on septic, ask how close the nearest Central San sewer main runs. Inside 300 feet changes what a future substantial remodel can require.
- Before budgeting an ADU or addition, get a straight answer on whether the project routes through Central San's connection fee schedule or through Contra Costa Environmental Health's septic and percolation review, since the approval timeline differs meaningfully between the two.
- If a new water meter is part of the plan, confirm whether the project can be served through the existing meter to avoid EBMUD's capacity charge.
- If you're selling a septic-lot property and considering a pre-listing renovation, price out whether a Central San Septic to Sewer conversion belongs in the plan before the addition, not after.
A Few Questions Worth Asking Early
How do I find out if a specific Alamo address is on septic or connected to public sewer? Contra Costa County's Environmental Health division and Central San's service records can confirm this for a given parcel, and it's worth verifying directly rather than assuming based on the neighborhood or the age of the home.
Does being on septic mean I can't build an ADU in Alamo? No. Septic-lot ADUs are approved regularly, but the path runs through Environmental Health's design and percolation review rather than Central San's connection fee schedule, and that review needs to happen before the county will sign off on the building permit.
Is the 300-foot sewer rule something the county enforces automatically, or only when I apply for a permit? It typically surfaces when a homeowner applies for a permit for a substantial improvement or when an existing septic system fails. It is not a background tax, but it is a real condition that can attach to a project once plans are submitted.
Renovation math in Alamo starts underground, not on the floor plan. If you're weighing an addition, an ADU, or a pre-listing renovation on a property you already own, or you're comparing two Alamo homes that look identical on paper, Julie Whitmer can help you get the septic-or-sewer answer before it becomes a surprise mid-project. Schedule a complimentary home strategy call to walk through what a specific Alamo property actually allows before you commit a budget to it.