The San Carlos Sewer Rule That Still Trips Up ADU Sellers

San Carlos sellers hear the reassuring version of this story constantly. Unlike San Mateo, Burlingame, Hillsborough, San Bruno, Millbrae, Pacifica, and South San Francisco, our city does not require a sewer lateral compliance certificate the moment you open escrow. That fact gets repeated at open houses and dropped into listing conversations as a point in San Carlos's favor, one less hoop before closing.

It is also the reason so many San Carlos sellers building an accessory dwelling unit get blindsided months before they ever list. The same inspection requirement comes back through a different door, one built into the remodel permit itself rather than the sale, and it lands with far less warning precisely because everyone has heard the no-point-of-sale-rule version and stopped there.

A construction rule, not a sale rule

San Carlos Municipal Code Chapter 13.05 governs private sewer laterals, the pipes that carry wastewater from a house to the public sewer main. The code does not put a certificate requirement on the transfer of title. Instead, it ties inspection to what the city calls a "building improvement," defined to include any of the following, unless a valid compliance certificate is already on file:

• Repair, remodeling, or improvement where the cost exceeds $75,000
• Repair, remodeling, or improvement affecting more than 25 percent of the building
• Repair, remodeling, or improvement where additional toilets are being installed
• A change of use from residential to commercial

Any one of those triggers a mandatory inspection, conducted by closed-circuit television or three-dimensional photography, performed by a licensed plumber or contractor, with results reported to the city's Director of Public Works. Pass, and the property gets a compliance certificate. Fail, and the city issues a notice to repair before the project can move forward.

None of that touches a straightforward resale. A seller who is not permitting any work can list, go to contract, and close without the city ever looking at the lateral. That is the accurate part of the reassurance.

Why every San Carlos ADU trips it anyway

Here is where the accurate version stops being useful. State law requires every accessory dwelling unit to include complete independent living facilities, and San Carlos's own zoning code spells out what that means at minimum: a sink, toilet, and shower. There is no version of an ADU in San Carlos that does not add a toilet.

That single fact means the "additional toilets" trigger in Chapter 13.05 fires on essentially every ADU project in the city, regardless of size, cost, or how it is otherwise permitted. A homeowner converting a garage into a modest studio, someone building a detached unit at the back of a larger lot, a family adding a unit specifically to create rental income before a future sale, all of them are installing a toilet where one did not exist. The trigger does not care whether the remodel costs $40,000 or $400,000. It cares whether a toilet is going in.

The same logic applies to a primary suite bathroom addition on the main house. The code trigger is written around toilets, not around ADUs specifically, so any project adding a bathroom anywhere on the property lands in the same category.

Why the timing catches people off guard

Because the trigger lives in the permit process rather than the closing process, the inspection happens during construction, not during escrow. San Carlos ties occupancy directly to it: an occupancy permit for the building improvement is only issued once the sewer lateral passes inspection. If the lateral fails, the project does not just need a repair estimate. It needs the repair completed, and often a fresh inspection, before anyone can legally occupy the finished ADU.

That creates a sequencing problem for anyone building an ADU with a sale in mind. A San Carlos seller who assumes the "no point-of-sale rule" means the lateral is a non-issue can find themselves near the end of a build, ready to list, with an occupancy hold they did not budget time or money for. The video inspection itself is only valid for six months from the date it is recorded, so a project that stalls in permitting can require a second look before the certificate is finalized.

The lateral itself remains the homeowner's responsibility to maintain at all times, certificate or not. What changes from project to project in San Carlos is simply whether the city requires proof of that condition before signing off on occupancy.

The market backdrop that makes this more common right now

This collision between ADU permitting and sale timing is showing up more often because more San Carlos sellers are adding ADUs before they list. The first half of 2026 brought the highest number of new single-family home listings the city has seen in five years, according to the White Oaks Blog's midyear recap, with total residential sales revenue reaching $426 million, the third-highest first-half total on record and just below the 2021 and 2022 peaks. Average and median home prices climbed back to near those record levels over the same period, with 140 homes changing hands through the first half of the year and only 17 percent of those purchases made in cash.

In a market where sellers are competing to stand out and buyers are stretching to compete for turnkey homes, adding income-producing square footage before listing has become a common play, particularly for the kind of upper-mid-market single-family homes that make up most of San Carlos's inventory. More ADU permits pulled means more sellers running into the toilet-installation trigger for the first time, often without realizing it applies until a contractor or the building division flags it.

What to actually budget for

Elsewhere in San Mateo County, where cities do run sewer lateral programs at the point of sale, testing typically runs a few hundred dollars, and repairs for laterals that fail can range from roughly $1,500 to $10,000 depending on the extent of the damage. San Carlos does not publish its own fee schedule for the 13.05 inspection separate from those regional ranges, so a seller planning an ADU should treat that broader range as the working estimate and confirm current numbers with the Building Division before finalizing a construction budget.

If you're planning a remodel or ADU in San Carlos:

1. Check whether the property already has a valid compliance certificate on file with the city before assuming an inspection is required.
2. Ask your contractor to fold the CCTV or 3D inspection into the earliest phase of the permit timeline, not the final walkthrough, so a failed result does not stall occupancy at the finish line.
3. Confirm the six-month validity window against your realistic construction schedule, since a slow permit process can outlast the initial inspection.
4. If the goal is to list soon after the ADU is finished, build the lateral inspection into your listing timeline the same way you would a final punch list item, not as an afterthought.

Common questions

Does replacing an existing toilet trigger the inspection? No. The city's own building permit guidance confirms that a like-for-like toilet replacement does not require a permit at all, and the Chapter 13.05 trigger is tied to the remodel permit for a building improvement, not routine fixture swaps.

Does this only apply to ADUs? No. The code trigger is written around any building improvement that adds toilets, exceeds $75,000, or affects more than a quarter of the structure. A primary suite bathroom addition on the main house falls under the same rule.

What if my property already has a compliance certificate? A valid certificate already on file with the city satisfies the requirement, and a new remodel permit for a different reason would not force a new inspection unless it independently meets one of the other triggers.

San Carlos's reputation for skipping a point-of-sale sewer certificate is accurate, and it is genuinely one less step for a seller who is not touching the plumbing. The moment a bathroom, especially an ADU bathroom, enters the plan, that reputation stops applying, and the inspection becomes a construction milestone that needs its own place on the timeline.

If you're weighing an ADU addition against a straight sale in San Carlos, or you want a realistic read on how either path affects your timeline and net proceeds, Ektra Real Estate can walk through the specifics with you. Get Your Home Valuation and let's map out what your property and your plans actually require before you pull a permit.