Two houses on the same road in Alamo can be running through two entirely different transactions, and price has nothing to do with it. One sits on the public sewer that Central Contra Costa Sanitary District, known locally as Central San, runs beneath most of town. The other, often a few hundred yards closer to the hills or tucked into the Westside, relies on a private septic system that answers to Contra Costa County's health department instead of Central San at all.
Most buyers don't think about which side of that line a house sits on until it changes something they actually care about: whether they can add the ADU they've been planning, what a pre-sale inspection has to cover, or whether they're writing a check to a plumber or taking on a fifteen-year charge that shows up on the property tax bill. Treat septic as a line item on a utilities checklist and you'll likely find out the hard way, usually mid-escrow.
The Line a Listing Sheet Won't Show You
The majority of Alamo connects to Central San's sewer grid. That's the water infrastructure most buyers assume applies everywhere in an established East Bay suburb. But a smaller set of parcels, concentrated in Westside Alamo and the foothill properties that climb toward Las Trampas and Mount Diablo, still run on private septic systems that predate the sewer lines reaching those pockets.
This isn't a quirk limited to older or lower-value homes. Some of the largest lots in town, the ones with the most acreage and the most flexibility for a pool, a guest house, or an ADU, are exactly the parcels most likely to be on septic. The systems that were adequate for a single-family home built decades ago are the same systems a buyer inherits today, bedroom count and all.
Two Different Regulators, Two Different Rulebooks
If a home is on public sewer, Central San is the agency that matters for anything involving the wastewater line. If a home is on septic, the agency is Contra Costa Environmental Health, part of the county's Land Use Program, which reviews site and soil evaluations, percolation testing, system design, and construction permits before anyone can install or modify a septic system.
The distance requirements alone show how granular this gets. County rules require a minimum of 100 feet between a well and a septic tank or leach field, a standard set specifically to protect groundwater that other properties nearby may also rely on for drinking water. The county's own health department treats septic in a suburban setting like Alamo as a workable but ultimately temporary solution, not a permanent fixture, which is part of why programs exist to help homeowners move off of it when a sewer main is close enough to reach.
For a buyer, this means the septic-versus-sewer question isn't really a single fact to disclose. It's a determination of which government office you'll be dealing with if anything changes on the property, and the two offices don't operate on the same timeline or the same rules.
The ADU Plan That Runs Into a Tank
Contra Costa County rewrote its ADU ordinance in 2025, and the update was generous to Alamo specifically. On lots of 12,000 square feet or more, which describes nearly every conventionally zoned parcel in town, homeowners can now build a detached accessory dwelling unit up to 1,200 square feet with a ministerial, formality-driven approval process. The old requirement that an ADU architecturally match the main house is gone.
That's the version of the story most buyers hear. The version that only surfaces once someone is deep into planning is that a septic lot doesn't get to skip a step the rest of town does. Septic systems are sized by bedroom count, so adding a bedroom or a full second unit can exceed what the existing tank and leach field were designed to handle. Before the county issues a building permit for that ADU, Contra Costa Environmental Health has to sign off on the septic system's capacity, and if the system can't support the addition, the upgrade becomes part of the project cost, not an afterthought.
| Step | Sewer-connected lot | Septic lot |
|---|---|---|
| Primary approving agency | Contra Costa County Department of Conservation and Development | Contra Costa County DCD, plus Contra Costa Environmental Health |
| What gets reviewed | Standard planning and building plan check | Same review, plus septic capacity for the added bedroom count |
| Extra sign-off before permit issues | None beyond standard review | Environmental Health approval of system capacity |
| Cost risk if capacity falls short | None tied to wastewater | Engineered system upgrade or expanded leach field |
For a buyer who's picturing an ADU as a rental unit or a place for aging parents, this is the difference between a straightforward permit and a project that starts with a septic capacity study.
What It Actually Costs to Cross the Line
For homeowners who decide converting from septic to sewer makes more sense than maintaining or upgrading a private system, Central San runs a Septic to Sewer program, known as S2S, for residential properties with an existing sewer main adjacent to the parcel. The program finances up to $65,000 in construction costs and Central San fees, repaid over 15 years through the Contra Costa County property tax roll at a published rate of 5.82 percent, a rate the district has indicated is due for review starting July 2027.
The connection itself carries separate fees, typically running $9,200 to $10,200 per residential unit depending on location, covering permit processing, annexation, capacity in the collection and treatment system, and a portion of the first year's sewer service charge. Properly abandoning the old septic tank, which the county requires rather than simply leaving it in the ground, adds a permit fee that was roughly $700 as of the district's most recent published guidance.
Here's the part that matters at resale. Because the S2S balance rides on the property tax bill rather than functioning like a private loan, it's worth confirming during escrow exactly how much remains and how it's structured, rather than assuming it gets paid off automatically at closing the way a home equity loan would. A seller who converted a few years into ownership may not have retired the balance yet, and a buyer needs to know what they're taking on before they assume the sewer connection is simply a completed upgrade with no strings attached.
Before You Sign Anything
California's Transfer Disclosure Statement and Seller Property Questionnaire require disclosure of known material defects, and a septic system's condition counts. Concealing a known issue, drainage problems, odors, a system that's been failing intermittently, exposes a seller to real legal risk after closing, not just an awkward conversation during negotiations.
A proper pre-sale septic inspection in California typically runs $300 to $900 and needs to come from a licensed C-42 contractor or a Registered Environmental Health Specialist, not just whoever last pumped the tank. Pumping and inspecting are not the same service. A real estate inspection includes a flow test, an examination of the tank and baffles, and a check for backflow from the drain field, the kind of detail that tells you whether a system is actually healthy rather than just recently emptied.
For sellers on septic in Alamo, getting that inspection done before listing, not after an offer comes in, is the difference between addressing an issue on your own timeline and discovering it during a contingency period with a buyer watching the clock.
A Few Questions Worth Asking Early
Does every home in Alamo use a septic system? No. Most of the town connects to Central San's public sewer. Septic is concentrated in Westside Alamo and the foothill parcels that predate the sewer lines reaching those areas.
How do I find out which system serves a specific address? Contra Costa Environmental Health maintains septic permit and system records, and Central San can confirm whether a parcel already connects to its sewer main. Both are worth checking before you write an offer, not after.
Can I add a bedroom or ADU if the home is on septic? Only if the existing system has the capacity to support it, since sizing is based on bedroom count. Contra Costa Environmental Health has to approve that capacity before the county issues a building permit for the addition.
Is converting from septic to sewer worth it? That depends on where you sit in your ownership timeline and how the conversion gets financed. Central San's S2S program makes the upfront cost more manageable, but the balance is collected through your property tax bill for 15 years, so it's a conversation to have with your agent and your escrow officer before you commit, not after.
If you're weighing a purchase or a sale on a septic parcel in Alamo, or you just want to know which side of the line a property sits on before you get attached to it, I'd rather help you find out now than have it surface during escrow. Kailani Kimoto works Alamo and the surrounding Contra Costa communities every week, and knows which questions to ask the county before they become your problem. Request Your Free Home Valuation and let's talk through what your specific property, and its wastewater system, actually means for your timeline.
