Los Altos Hills Is Rewriting Its ADU Rules. What Does a Backyard Unit Actually Add to a Lot?
October 15, 2026
A finished backyard unit on a Los Altos Hills property can hold square footage the main house is not allowed to have. Town code lets an accessory dwelling unit go 800 square feet past a lot's maximum development area and another 800 square feet past its maximum floor area. Under current local rules, the unit cannot be sold apart from the house, with a narrow exception for qualifying units built by nonprofits. Taken together, those two rules make a typical ADU here something other than a second asset on the lot. It is extra room that only the owner of the main house can hold, and its value moves with the main house.
The timing matters. On September 14, 2026, the Los Altos Hills Planning Commission held a special workshop on "potential updates to the Town's ADU regulations." A buyer comparing large-lot towns this fall is reading rules the Town has formally reopened.
Every Los Altos Hills lot has two ceilings: a maximum development area and a maximum floor area. On a property already built to those limits, the main house cannot grow any further. An ADU still can, up to the 800-square-foot allowance. Any ADU area beyond that allowance counts against the same limits as everything else on the lot.
The current text of Article 14 of the municipal code, published as of October 4, 2026 in the GoCodebook mirror of the Los Altos Hills code, sets these limits:
| Unit type | Size limit | Height |
|---|---|---|
| Detached ADU, studio or one-bedroom | 850 sq ft | Generally one story, 16 ft |
| Detached ADU, more than one bedroom | 1,000 sq ft | Generally one story, 16 ft |
| Attached ADU | Lesser of 1,000 sq ft or 50% of the main home's living area, with at least 800 sq ft allowed | Same as the primary dwelling |
| Junior ADU | 500 sq ft | Within the home |
| Detached ADU with 30-ft side and rear setbacks plus one extra parking space | Up to 1,200 sq ft, subject to the ordinary area limits | One story, 19 ft |
A detached unit may reach 18 feet within a half-mile walk of a major transit stop or high-quality transit corridor, or 20 feet where needed to match the main home's roof pitch. Each single-family lot may have one ADU and one junior ADU. The current text does not provide for more than one detached ADU on a single-family lot.
There is one more exemption that matters to a buyer. The size caps do not apply to an ADU created by converting existing floor area inside an existing structure. So the same house can offer two very different options. One route builds a new detached unit and uses the 800-square-foot allowance. The other converts space that already exists, such as a pool house, a wing or a large garage, and avoids the size caps.
Under Article 14, an ADU or junior ADU in Los Altos Hills may be rented separately, though not for less than 31 days. It may not be sold or conveyed separately, except for qualifying units built by nonprofits that meet state-law conditions. The code does not require the owner to live on the property for a standard ADU. The state's March 2026 ADU Handbook says local agencies generally cannot impose that requirement, apart from a statutory exception tied to certain separately conveyed units.
State law now opens another path, but each city has to choose to use it. AB 1033, codified in Government Code §§ 66340 through 66342, lets a jurisdiction allow ADUs to be sold as condominiums, subject to conditions, if it adopts a local ordinance first. The draft minutes of the Town's Housing Element Implementation Committee meeting on September 14, 2026, which call the law "SB1033," record that Los Altos Hills has not adopted such an ordinance.
The law can be used. Palo Alto Online reported that in June 2026 an ADU in San Jose became the first in California to be sold separately from its main house, closing at $530,000. In the same article, Palo Alto's Planning and Transportation Commission voted on September 9 against recommending separate sale to the City Council. Commissioner Allen Akin said, "I don't see a huge win in creating an entirely new mechanism with comparatively few advantages over SB 9." Two commissioners, Bart Hechtman and Cari Templeton, appeared sympathetic to the idea.
For a Los Altos Hills buyer, the result is simple. A finished ADU's value comes from what it does for whoever owns the main house: rental income on leases of 31 days or longer, room for family, or flexible space. Outside the nonprofit exception, there is no local route to selling that value off on its own. When a listing includes a unit, the unit's value is priced into the house.
Three documents from the past two years point the same way:
The gap between the first two items may come down to what was submitted to HCD rather than what the Town adopted. The Town's own code history lists Ordinance 612 as an amendment to Article 14. The workshop agenda listed no specific proposed changes and set no follow-up hearing date. A buyer with escrow closing this fall is buying under today's Article 14. A buyer planning to build next year may be working under a revised version.
The Town's housing plan explains the attention. According to the draft committee minutes, the state requires Los Altos Hills to plan for 489 units in the current Housing Element cycle. The plan counts on ADUs for 160 of them, more than any other housing type, compared with 32 from SB 9 projects with their associated ADUs and 48 from single-family subdivisions.
The Town had permitted 146 units through 2025, which leaves 343 still to be created, 146 of them affordable. Community Development Director Jay Bradford reported current production of 30 to 35 units a year. Figures for 2026 are not yet available. At that pace, the remaining 343 units would take roughly ten years or more to produce.
The Town's other routes are larger and slower. Three sites have been rezoned for multifamily housing: Twin Oaks Court, St. Nicholas School and Foothill College. The minutes list them together at 377 units on 19 acres, and the Town is still in talks with owners and looking for developers. A program called A12 would allow an existing single-family home to be converted into two or three units in R-A areas with lower fire hazard and adequate utilities. It would need new objective design standards first. Of all these options, ADUs on existing lots are the one homeowners can add on their own.
Three questions help separate one Los Altos Hills property from another:
For sellers, these points describe what a lot can still become. For buyers, they explain why two homes of similar size on similar acreage may offer very different room to grow. Since separate sale is generally not available here, that potential is priced into the main property.
Can I buy a Los Altos Hills ADU on its own? Generally not under current local rules. Article 14 prohibits separate conveyance except for qualifying nonprofit-developed units. The Town has not adopted the local ordinance that state law requires before ADUs can be sold as condominiums.
Can an ADU be rented? Yes, for periods of 31 days or longer.
Are pre-designed plans available? Los Altos Hills is one of the communities in the Santa Clara County ADU Plans Gallery, which the Los Altos Town Crier described as a tool for browsing pre-designed and locally pre-approved plans.
Will the rules change? The Planning Commission has opened the question but has not published proposed amendments or a hearing date. The Town's planning department has the current requirements for any specific property.
If you are weighing a Los Altos Hills property where an existing unit or the 800-square-foot allowance could shape what the lot is worth to you, Yvette Stout can review its floor-area position and conversion options with you before you write an offer. Schedule a private consultation to discuss the property and a strategy suited to it.
This article is general information, not legal, tax or financial advice. Consult a qualified professional about your situation.
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