The Real Limit on a Los Altos Hills ADU Isn't the Zoning Code. It's the Septic Tank.

In Los Altos Hills, the number everyone quotes for accessory dwelling unit size comes from the Town's own municipal code: 850 square feet for a studio or one-bedroom detached unit, 1,000 square feet for anything larger, and up to 1,000 square feet or half the primary home's living area if the unit is attached. That number is accurate. It is also, for a large share of Hills properties, not the number that ends up deciding what gets built.

The Hills sit mostly outside the municipal sewer system. Many parcels run on private septic systems that predate current design standards, and before the Town issues a building permit for an ADU, Santa Clara County's Department of Environmental Health has to independently confirm that the existing septic system can handle the added wastewater load. That review runs on its own logic, tied to soil percolation rates and bedroom counts rather than square footage, and it can arrive at a smaller number than the zoning code ever suggested.

What the Zoning Code Actually Allows

The Town's ADU ordinance is generous by Bay Area standards, which is part of why the septic constraint catches people off guard. One ADU and one Junior ADU are allowed on any lot in the R-A district with an existing single-family home, regardless of lot size. The size caps break down by type:

Setbacks add another layer. A new ADU needs a front setback of at least 40 feet and side or rear setbacks of at least 4 feet, with a base height limit of 16 feet. Give the unit a 30-foot side and rear setback instead, and it qualifies for additional floor area and a height bump to 19 feet. One onsite parking space is required unless the ADU sits within a half mile of a public transit stop or was created by converting existing floor area rather than adding new square footage.

Read only this far, a buyer evaluating a Hills property for guest house potential would reasonably conclude the ceiling is whatever the zoning table allows, minus whatever the setbacks take away. That conclusion holds only if the property is connected to a public sewer main. For everyone else, there is a second approval waiting after the zoning math is settled.

The Approval That Runs on a Different Formula

Santa Clara County's Department of Environmental Health treats septic capacity as a wastewater engineering question, not a zoning one. Before a homeowner can connect a new ADU to an existing system, the county requires a site plan and, if the existing septic will be used, a current septic pumper's report from within the last three years that includes a 30-minute water test. An inspector then conducts an initial evaluation, which can include a site visit to confirm setbacks and verify there is enough confirmed expansion area for the drainfield. If soil conditions are uncertain, additional testing follows before anyone gets an answer.

The county's own guidance on the process is direct about the underlying math: drainfields are sized based on the percolation rate of the soil and the number of bedrooms in each structure on the property, and the additional wastewater load from a secondary dwelling has to be evaluated on that basis, with drainfields expanded if the existing system was not designed to carry it. A system built decades ago for a modest ranch house was sized for that house's bedroom count. Adding a two-bedroom ADU on top of it is not a floor-area question. It is a load question, and the load question gets answered by the county, not by the zoning table.

If the existing septic system meets current standards and has room to spare, Environmental Health issues what the county calls an approval-recommended stamp, and the project moves forward. If it does not, the owner has to upgrade or redesign the system before the ADU application can proceed, which means a separate design submission, a separate review, and a separate timeline layered on top of the Town's own permit process.

Why an Old Tank Can Cap a New Room

The Town's own septic bulletin describes what a compliant system looks like: a concrete tank where possible, two drainfields each sized to handle half the total load and interconnected through an approved diversion valve, plus a reserve area equal to at least 100% of the disposal field's current size held in reserve for future expansion. Systems installed before these standards existed do not automatically fail, but they do not automatically pass either. A property with a 1970s-era single-drainfield system, no diversion valve, and no reserve area on file may need a full redesign before it can legally support one more bedroom, regardless of what the ADU ordinance would otherwise permit on that lot.

This is the detail that gets lost when a listing or a buyer's search treats the zoning cap as the whole answer. Two lots with identical R-A zoning and identical square footage can have very different real-world ADU potential, and the difference lives in a county file most people never think to request until an offer is already in.

The Calendar Adds Its Own Constraint

Even a septic system with room to grow runs into scheduling limits that have nothing to do with permitting queues. Installation and repair work is prohibited between November 1 and April 1 in Los Altos Hills, with emergency repairs the only exception, and those require an approved erosion control plan. Drainfields are capped at a 50% slope, and anything over 20% triggers additional investigation before the county will sign off. On a hillside lot, that combination of seasonal restriction and slope-triggered review can push a straightforward-looking project into a second construction season before ground is ever broken.

What This Costs When the Answer Comes Back No

Recent Los Altos Hills-specific contractor estimates put engineered septic systems, the kind often required when an older system can't be expanded to standard, in the range of $30,000 to $80,000 or more, depending on soil conditions and system type. That range sits on top of whatever the ADU itself costs to design and build. It is not the number that derails a Hills purchase on its own. What derails a purchase, or stalls a listing, is discovering that number exists after the inspection contingency has already closed, or after a buyer has already built an ADU budget around the zoning table's larger square footage without knowing the septic system had a smaller number in mind.

Before You Write the Offer or List the Property

The fix is simple and almost entirely a matter of asking for documents before, rather than after, a deadline is running.

• Request the existing septic as-built and any prior county correspondence about the system's approved bedroom count and design capacity.
• Order a current septic pumper's report and 30-minute water test if the property doesn't already have one dated within the last three years. The county requires this before it will even begin an ADU evaluation.
• Ask whether Environmental Health has already reviewed the system for the ADU size under discussion, or whether that review has not yet started. A stamped, approval-recommended site plan is worth far more than a verbal assurance that "the septic should be fine."
• Confirm whether a public sewer main runs within 200 feet of the parcel. Town code requires connection to the main rather than continued septic use in that case, which changes the entire calculation.

For a seller, having these documents ready before listing turns a potential objection into a settled fact. For a buyer, having them before the inspection contingency expires is the difference between negotiating from information and negotiating from guesswork.

Quick Answers

Does Santa Clara County require a septic inspection just to sell a home in Los Altos Hills, with no ADU involved? Not automatically. The county's review process is triggered by a building permit application, whether for an ADU, an addition, or a new accessory structure, not by the sale itself. A system that hasn't been touched in years can still surface the moment any project starts.

Can I still build an ADU if my septic system is decades old? Often, yes, but the county will evaluate whether it meets current standards, including a concrete tank, a diversion valve, and two interconnected drainfields with a documented reserve area, before approving any added bedroom load. An older system that predates those standards may need upgrading regardless of how much floor area the zoning code would otherwise allow.

What if my lot is near a sewer line instead of relying on septic? If a public sewer main runs within 200 feet of the parcel, Town code requires connecting to it rather than maintaining a septic system, which removes the county wastewater review from the equation entirely.

If you're weighing whether a Los Altos Hills property can actually support the guest house, caregiver suite, or income unit you have in mind, that answer starts in the septic file, not the floor plan. Vicki Ferrando works with Hills buyers and sellers to pull the right county records before contingencies expire, not after.