Alamo has no city hall. Every permit, every septic determination, every question about whether a parcel can support a guest house runs through Contra Costa County, because Alamo has never incorporated. That single fact explains more about the town's plumbing than any zoning map, and it explains why a meaningful number of Alamo's older, larger lots still run on private septic systems decades after sewer mains reached the street in front of them.
The Central Contra Costa Sanitary District, the utility that would take those homes off septic if asked, says plainly on its own site that septic properties are less marketable than comparable sewered homes at time of sale, and that a failing system will actively depress value until it's repaired, replaced, or converted. That's not a listing agent's opinion or a home inspector's caution. It's the sewer district's own framing of the asset in front of it. If you own or are considering a septic-served property in Alamo, that sentence is the whole conversation, and everything else in this post is what to do about it.
Why Septic Never Fully Left Alamo
Alamo sits under Contra Costa County's Department of Conservation and Development for planning and building, and under Contra Costa Health's Environmental Health Division for anything touching a septic system or a well. There is no Alamo planning office to call. The county's Application and Permit Center at 30 Muir Road in Martinez wears every hat, and Environmental Health can be reached directly at 925-608-5500 for questions specific to a parcel's septic status.
Central San's own service area list names Alamo alongside Danville, Lafayette, Moraga, Orinda, Pleasant Hill, Walnut Creek, and parts of Martinez and San Ramon as communities where private septic systems still coexist with public sewer mains. The reason isn't neglect. It's geometry. Large lots mean long distances between a house and the nearest public main, and connecting a house at the back of an acre-plus parcel has always cost more than connecting one on a standard subdivision lot. A local plumbing contractor serving Alamo's estate streets, from the ranch-style properties along Stone Valley Road to the equestrian lots near Livorna Road and the homes around Round Hill Country Club, notes that large lots mean long sewer runs, which is exactly the cost driver that has kept some of these systems in place long after the surrounding town connected.
The Rule That Forces the Question Even If Your System Is Fine
Most owners assume a septic system only becomes a problem when it fails. Contra Costa Environmental Health's own guidance says otherwise. If a parcel sits within 300 feet of an existing Central San sewer main, the county can require the owner to connect to public sewer under two separate conditions: when the existing system fails, or when the owner plans substantial improvements to the home. Failure and renovation are treated as the same trigger.
That second condition is the one that catches people off guard. A kitchen remodel, a second story addition, or an ADU application can force a conversion decision that has nothing to do with whether the septic system is actually working. The county doesn't wait for a crisis. It uses any major project as the moment to ask whether the property should still be running its own private treatment system a few hundred feet from a public main built to handle exactly that load.
| Trigger | What Happens | Who Decides |
|---|---|---|
| System failure | CCEH evaluates whether repair, an engineered replacement, or sewer connection is required | Contra Costa Environmental Health |
| Substantial home improvement | If within 300 feet of a main, the county can require sewer connection as a condition of the permit | Contra Costa Environmental Health |
| Voluntary conversion | Owner applies to Central San for a sewer permit and, if eligible, financing | Central San |
What the Sewer District Says About Value
Central San's septic conversions page is clear on the resale question. It states that converting from a private septic system to public sewer may increase a property's market value, and that a failing septic system can decrease market value until the owner resolves it through repair or connection. It goes further, noting that because septic systems are unpopular with most homebuyers and can limit the ability to expand a house or build over parts of the lot, a septic property will be less marketable at time of sale compared to a similar sewered property.
That's the sanitary district describing its own service area's real estate, not a competing pitch. For a seller weighing whether to convert before listing or disclose and let the buyer factor it into an offer, that's the baseline fact to work from.
The Financing Program Built for This Exact Gap
Central San runs a Septic to Sewer program, known as S2S, specifically for residential owners with an existing sewer main adjacent to their parcel who want to convert. The program finances up to $65,000 of the conversion costs, covering Central San's permitting, inspection, and capacity fees along with the construction and septic tank abandonment work. Repayment happens over 15 years through the Contra Costa County property tax bill rather than a separate loan payment, which keeps the carrying cost predictable for an owner who plans to stay. The interest rate is set periodically by the district's board, so anyone budgeting a conversion should confirm the current rate directly with Central San's Permit Desk at 925-229-7371 rather than relying on a figure that may be a few months stale.
The process starts with confirming eligibility (an adjacent sewer main), locating the existing septic tank, and getting a contractor bid approved by Central San before the financing paperwork is executed. A separate abandonment permit from Contra Costa Environmental Health is required to properly retire the old tank, and that fee has historically run a few hundred dollars, though owners should verify the current amount with CCEH before finalizing a budget.
Why This Matters More If You're Adding an ADU
Alamo's large lots have made it one of the more active ADU markets in central Contra Costa since the county rewrote its accessory dwelling ordinance in 2025. The rewrite dropped the old requirement that an ADU architecturally match the main house, and on any conventionally zoned Alamo lot of 12,000 square feet or more, a detached unit can run up to 1,200 square feet with an attached garage that doesn't count against that size.
None of that helps if the wastewater question hasn't been answered. On a sewered lot, Central San charges an ADU capacity fee of $3.63 per square foot, capped at 1,200 square feet, which works out to under $3,000 for an 800-square-foot unit. On a septic lot, the county's Environmental Health division has to approve the existing system's capacity before the ADU can be established at all, and whether an aging system can absorb another bedroom or two is a site-specific engineering question, not a formality. Where the answer comes back no, the S2S program is often the practical path forward, since it can finance the public sewer connection that makes the ADU possible in the first place.
For a seller marketing a large Alamo lot's potential for a guest house or rental unit, the septic-or-sewer question isn't background information. It's the first thing a serious buyer's contractor will ask.
What This Means If You're Selling or Buying
If you're preparing to list a septic-served property in Alamo, the sequence that protects value looks like this. Confirm with Contra Costa Environmental Health whether your parcel falls within that 300-foot radius of an existing main. If it does, decide before listing whether you want to convert through Central San's S2S program and pass along a sewered property, or disclose the septic system's condition and let the market price it accordingly. Order a professional inspection early enough to address any findings on your own timeline rather than the buyer's, since lenders financing FHA or VA loans commonly require septic verification regardless of what the county mandates.
If you're buying a septic-served property in Alamo, ask the same 300-foot question before you get attached to the house. A system that works fine today can still trigger a mandatory connection the moment you pull a permit for the addition that made you want the house in the first place.
Frequently Asked Questions
Does Contra Costa County require a septic inspection every time an Alamo home sells? The county doesn't mandate a blanket inspection at every transfer. What it does enforce is the 300-foot rule tied to system failure or substantial improvements, and separately, many lenders on FHA and VA loans require their own septic inspection as a condition of financing.
How do I find out if my Alamo property is within 300 feet of a Central San sewer main? Contra Costa Environmental Health at 925-608-5500 can confirm this for a specific parcel, and Central San's permit counter can also verify main proximity if you're evaluating a conversion.
Is the Septic to Sewer program only for Alamo? No. It's open to residential owners across Central San's service area, which includes Alamo, Danville, Lafayette, Moraga, Orinda, Pleasant Hill, Walnut Creek, and parts of Martinez and San Ramon, as long as an existing sewer main runs adjacent to the parcel.
Does converting to sewer guarantee a higher sale price? Central San states that conversion may increase market value and that a failing septic system can decrease it, but the district doesn't publish a specific dollar figure. The stronger, better-documented claim is the comparative one: a septic property is less marketable than an otherwise similar sewered property.
If you're weighing whether to convert before listing, disclose and price accordingly, or check what a septic system means for an ADU you're planning, Kory Madge can walk through the county and district steps with you and bring in the right contractors before a single showing happens.
