"This is revenue we need. I mean, we deserve it."
That's Roy Thiele-Sardiña, treasurer of the West Bay Sanitary District board, explaining why the district spent this year sending letters to roughly 180 Atherton property owners asserting that they owe sewer charges, in some cases going back years, for accessory structures the district believes were never properly billed. If you own a home in Atherton with a guest house, a pool cabana, or a converted garage, that sentence is worth sitting with for a moment. The bill isn't about your water use. It's about a bureaucratic gap between two record systems that never quite matched up, and it's arriving at exactly the moment a lot of Atherton owners are thinking about their next move.
Two Record Systems That Don't Talk to Each Other
The mechanics are almost mundane, which is what makes them dangerous. West Bay Sanitary District, the special district that handles wastewater for most of Atherton, built its audit primarily from County Assessor records rather than the Town's own building permit files. The Town has said publicly that this approach has produced a specific and predictable error: garages, gazebos, pool houses, and cabanas are being flagged as accessory dwelling units when they were never permitted, built, or used as one. A structure with a sink and a half bath is not the same thing as a legal ADU with a kitchen and independent living space, but an assessor's record does not always draw that line the way a building permit does.
The confusion has a real cause behind it. California's 2026 ADU rules require local agencies to process ADU and JADU permits ministerially, on a fast, largely administrative track. That speed is good for housing supply, but it means a county can approve and finalize an ADU permit without continually looping in the sanitary district that actually has to carry the additional wastewater flow. A 2023 San Mateo County civil grand jury report found that towns including Atherton, Portola Valley, and Woodside lean on ADUs for as much as 80 percent of their state-mandated housing targets. That reliance is a policy success on paper. It also means a meaningful number of backyard units in this part of the Peninsula were built and occupied without ever generating a sewer service account, which is the gap the district is now trying to close all at once, retroactively, at the address level.
Two Districts, and Whether This Even Applies to You
Not every Atherton parcel sits inside West Bay Sanitary District's territory. The district's Atherton coverage runs roughly through the area between Atherton Avenue and Marsh Road on one side and Valparaiso on the other, bounded by Highway 280 and Bay Road. The remainder of town is served by the Fair Oaks Sewer District, which is governed directly by the San Mateo County Board of Supervisors rather than by West Bay's own elected board. If you're weighing whether this audit touches a specific address, the first question isn't whether the home has an accessory structure. It's which sewer district actually serves that parcel, because the audit and the billing dispute described here are a West Bay Sanitary District matter, not a Fair Oaks one.
What Changed Between the First Letters and Now
The letters that went out this spring did not stay static. Reporting on the dispute in December 2025 described a district asking for five years of retroactive sewer charges on flagged properties, even though residents and the Town pointed to a three-year statute of limitations as the more defensible standard. That disagreement went to the board directly. On May 13, 2026, the board voted to seek three years of unpaid sewer service charges instead of five, and to treat January 1, 2020 as the effective date for calculating any unpaid connection fees. The board also confirmed the collection mechanism that gives this issue its teeth for a home sale: if a property owner does not pay a charge the district asserts is owed, that amount can be added to the county tax roll as a delinquency following a public hearing.
That last detail is the one worth sitting with if you're preparing to list a home or already in contract on one. A tax roll delinquency is not a polite invoice. It attaches to the parcel in the same records a title company checks, on the same public record a buyer's lender will review before funding.
| Date | What happened |
|---|---|
| December 2025 | Local reporting describes West Bay Sanitary District pursuing five years of retroactive charges on flagged Atherton properties |
| Spring 2026 | Roughly 180 letters go out to Atherton property owners asserting sewer service and connection charges are owed |
| May 13, 2026 | Board votes to limit the retroactive lookback to three years and sets January 1, 2020 as the effective date for unpaid connection fees |
| May 27, 2026 | District board holds its next regular meeting, opened to public comment, as Atherton residents plan to attend over the letters |
Why This Surfaces at the Worst Possible Moment
Atherton is not a market where anyone can afford a slow escrow. Sale prices over the three months ending in July 2026 ran well into eight figures, per Redfin, and homes were moving to pending in around 17 days, with hot listings closing even faster. A district letter that arrives after a home is already in escrow, asserting a charge that could later land on the tax roll, is exactly the kind of surprise that stalls a fast-moving deal or gets renegotiated into the closing statement at the seller's expense.
The structural risk is that a charge like this does not necessarily show up on a preliminary title report the way a recorded lien would, because it may not yet be on the tax roll when escrow opens. It becomes visible only after the district's process runs its course, which means the safest posture for an Atherton seller is to resolve any ambiguity before the property ever goes on the market, not after a buyer's inspector notices a pool house with a bathroom and starts asking questions.
What to Check Before You List or Make an Offer
- Confirm which sewer district actually serves the address, West Bay Sanitary District or Fair Oaks, since the audit only applies to the former.
- Pull the Town's permit history for any accessory structure on the property, using the building department's own records rather than relying on what the county assessor has on file.
- Establish whether any structure was ever permitted as a true ADU, with independent kitchen and living facilities, versus a non-qualifying accessory building like a garage, gazebo, pool house, or cabana.
- If a letter from West Bay Sanitary District has already arrived and no ADU exists on the property, contact the district directly and reference the Town's permit records as supporting documentation before any charge advances toward the tax roll.
- If a legitimate ADU does exist and was never connected through the district's own review process, plan for a connection fee calculation that uses January 1, 2020 as the retroactive starting point, plus up to three years of back sewer service charges.
The Town of Atherton has told residents who receive a letter in error to contact West Bay Sanitary District directly and to lean on the building department's own permit files as proof, since the Town's Building Official can help pull that history for a specific parcel.
A Short FAQ
Does this affect every home in Atherton? No. It applies specifically to parcels served by West Bay Sanitary District. Homes served by the Fair Oaks Sewer District, which is governed by the San Mateo County Board of Supervisors, sit outside this particular dispute.
What actually counts as an ADU versus a pool house? The distinction the Town has drawn publicly is functional. A structure with permanent provisions for independent living, meaning cooking, sleeping, and sanitation, is an ADU. A garage, gazebo, cabana, or pool house is not, even if it has plumbing, unless it was specifically permitted to meet ADU requirements.
I already received a letter and don't have an ADU. What now? Contact the district directly and promptly, and bring the Town's permit records for the property as documentation. The Town has said the district's methodology has produced misidentifications, so a paper trail from the building department is the most direct way to correct the record before a disputed charge moves further along.
Atherton's sewer geography, permit history, and now this audit all live in the kind of local detail that doesn't show up on a listing sheet. If you're weighing when to list, or you're under contract on a property with any kind of accessory structure, it's worth a conversation before those questions surface in someone else's timeline. Hebe Li works this market closely enough to know which of these details matter for your specific address. Let's Connect.
