Atherton · San Mateo County
Custom Homes, Guest Houses & Estate ADUs in Atherton, California
Atherton's design standards and large estates call for an architect-led approach. SHAPE delivers it with a fixed price.
Building custom homes and estate ADUs in Atherton, California
SHAPE designs and builds custom homes, guest houses, and estate ADUs for owners in Atherton, California , from Lindenwood, West Atherton, Lloyden Park, and the estate blocks off Selby, Isabella, and Fair Oaks.
Atherton is one of the most exclusive residential zip codes in America. Owners here don't need rental income, they need guest houses, pool houses, and estate ADUs that match the caliber of the main residence.
We're a licensed California general contractor (CSLB #1158688) with an architect-led, design-build model. One team owns the drawings, the permits, and the field work, so a Atherton project moves without the usual finger-pointing between architect, contractor, and city.
Atherton Building Standards at a Glance
- Setbacks: California state law caps rear and side setbacks for new ADUs at 4 feet, and Atherton follows that standard.
- Height: Detached ADUs can be 16 feet tall, or 18 feet within a half-mile of transit. An ADU attached to the house or built over a garage can reach 25 feet, and Atherton allows ADUs up to two stories. Upper-floor ADUs need privacy screening and downward-facing exterior lights.
- Maximum size: Atherton allows an ADU and a junior ADU on a single-family lot, with detached ADUs up to 1,200 sq ft. Lots of 2 acres or more can add a bonus ADU if it is deed-restricted as affordable housing.
- JADUs: Junior ADUs (up to 500 sq ft) are allowed inside the existing footprint of the primary home, in addition to a detached ADU on most single-family lots.
- Parking: No replacement parking is required when a garage or carport is converted or torn down to build an ADU, wherever the lot is. Separately, no parking can be required for the ADU itself within one half-mile walking distance of public transit.
- Sewer and water: West Bay Sanitary District or the Fair Oaks sewer district, plus Cal Water and Menlo Fire fees. West Bay Sanitary has moved to bill ADUs it was never told about, so we register the ADU with the district from the start.
Rules checked September 2026 against: Atherton Municipal Code 17.52.040, Atherton Municipal Code 17.40.040, Atherton Municipal Code 17.60.020, Town of Atherton ADU page, San Mateo County ADU Resource Center, Atherton tree protection, HCD ADU Handbook, California Energy Commission ADU guidance, San Mateo County Assessor, California Department of Insurance, Safer from Wildfires. Town rules change often. We confirm the rules for your lot before design.
SHAPE handles the full permit package with the Atherton planning department, zoning verification, building plans, Title 24 energy, and every correction cycle, so you don't have to learn the code.
Our Process
- Site assessment. We walk your Atherton lot, confirm setbacks and utility locations, and give you a feasibility read and an all-in price the same week.
- Design. Our in-house architects design the custom home or estate ADU around your lot, your family, and your budget, with fixed pricing before you sign.
- Permits. We submit, respond to every planning and building comment, and drive the file to permit issuance with Atherton.
- Build. Modules are fabricated in our facility while site work, foundation, utilities, landscaping, runs in parallel. Set and finish happen on site.
Guest house or ADU in Atherton?
Atherton is one of the few towns where a guest house can have a kitchen. The catch is that a guest house can be occupied only 30 days a year without a conditional use permit, it counts toward floor area, and all accessory buildings together are capped at 1,200 square feet per acre. An ADU can be lived in full time, and its first 800 square feet do not count toward floor area.
| Guest house | ADU | |
|---|---|---|
| Use | Up to 30 days a year without a use permit | Full time |
| Floor area | Counts. All accessory buildings capped at 1,200 sq ft per acre. | First 800 sq ft excluded |
| Height | 15 feet, one story | 16 feet detached, 25 feet over a garage, up to two stories |
| Side and rear setbacks | 10 feet | 4 feet up to 800 sq ft |
Unless the building will only host occasional guests, an ADU gives you more room and more use.
Heritage oaks and tree protection in Atherton
Valley, coast live and blue oaks measuring 48 inches around or more are heritage trees in Atherton, and their protection zones can extend onto neighboring lots. Applications need a tree preservation plan. During construction the Town expects 6-foot chain-link fencing around protected trees, regular arborist reports and about two weeks of Town Arborist review. We map heritage trees before siting the building, because that usually decides where a guest house can go.
Costs beyond the building on an Atherton lot
In Atherton the outside costs come mostly from the sewer district and PG&E. We price the ones your lot needs before you sign.
| Item | Amount |
|---|---|
| Sewer connection, West Bay Sanitary District | $8,608 per equivalent dwelling unit |
| West Bay Sanitary annual sewer charge per dwelling (2025 to 2026) | $1,440 |
| PG&E new electric service for an ADU (average, projects finished 2023 to August 2025) | $4,832 |
| PG&E new gas service for an ADU (same period) | $16,865 |
Sources: West Bay Sanitary District, PG&E ADU service report
Timeline for a Atherton ADU
In Atherton, tree protection and outside agencies run alongside the Town permit. Here is how the clocks line up.
| Step | Typical time |
|---|---|
| Town completeness check | 15 days under state law (2026) |
| Town decision on a complete application | Within 60 days under state law |
| Town Arborist review of the tree protection plan | About two weeks |
| PG&E new service | Averaged 118 days as of March 2026, by PG&E's count |
| Build, from permit | 2 to 4 months with our modular process |
We submit the tree preservation plan and the PG&E application with the permit so neither holds up the build.
Sources: 2025 ADU law update (SB 543), PG&E new service times
Frequently asked
- How much does a custom home or estate ADU cost in Atherton?
- Custom homes and estate-caliber ADUs in Atherton typically start in the mid-seven figures for a full ground-up build and from the mid-$500ks for detached guest houses. SHAPE quotes a fixed all-in number after a site walk and a design brief.
- How long do permits take in Atherton?
- Atherton's design-review-heavy process usually runs 6–12 months for a full custom home, and 2–4 months for an ADU under California's ministerial review rules. SHAPE manages every submittal and correction cycle.
- Do I need to add parking?
- Atherton custom homes follow the local off-street parking standards. For ADUs, no replacement parking is required when a garage or carport is converted or torn down to build the ADU, and no parking can be required for the ADU itself within a half-mile walking distance of public transit.
- Can I build a guest house and an ADU on the same lot?
- In most cases yes, Atherton allows one ADU (and often a JADU inside the main house) per single-family lot in addition to accessory buildings that don't meet the ADU definition. SHAPE lays this out on the site plan before design starts.
- Do you work with an existing architect?
- We do. SHAPE can execute a set of drawings your architect has already developed, or run the full design in-house. Either way, one contract and one fixed price.
- Can I build an ADU on any Atherton lot?
- Yes, on any lot with an existing or proposed single-family home, and state law does not allow a minimum lot size for ADUs. On large Atherton lots the questions are placement, trees and floor area, not eligibility.
- Where on my lot can a 1,200 square foot guest house go?
- An ADU of 800 square feet or less can sit 4 feet from side and rear lines. Larger detached ADUs follow accessory building setbacks, 10 feet from side and rear lines, and the San Mateo County ADU Resource Center notes they sit 120 feet from the front line or 30 feet behind the front of the house.
- Does an Atherton ADU go through design review?
- No. Atherton approves ADUs through staff review, and the San Mateo County ADU Resource Center lists a decision within 60 days. Design review is not part of the ADU path.
- What counts as a kitchen in an Atherton guest house?
- Atherton's code defines a kitchen by a full set of cooking appliances in a room of at least 80 square feet, and it allows one kitchen in an approved accessory building. The limit is on use: occupying a guest house more than 30 days a year needs a conditional use permit.
- How many bedrooms can an Atherton ADU have?
- State law does not let towns cap the number of bedrooms in an ADU. The size limits and your floor area decide how many fit.
- Are fire sprinklers required in an Atherton ADU?
- Only if the main house is required to have them. Atherton is outside the high fire hazard zones on the 2025 state maps, and Menlo Park Fire has said most ADUs are exempt from sprinklers.
- Does an Atherton ADU need solar panels?
- A new detached ADU does. Under the 2025 California Energy Code it needs new solar panels, which can be added to an existing system elsewhere on the property instead of the ADU roof. An attached ADU counts as an addition and does not need solar.
- Does the ADU need its own utility connections?
- Not always. State law bars a separate-connection requirement for ADUs built inside existing space. For a new detached ADU, PG&E, Cal Water or the sewer district can require its own connection, with fees in proportion to the ADU's size. We settle this with each utility during design.
- Can I build the ADU and a new main house at the same time?
- Yes. State law lets the Town approve both together, and the ADU is still reviewed without discretionary review. When the two are built together, utilities can treat the ADU as new residential use for connection fees.
- Will an ADU raise my property taxes in Atherton?
- Only on the new building. The San Mateo County Assessor does not reassess the existing home. It adds the ADU's market value at completion to your current assessed value, with a partial assessment if the work is only partly done on January 1.
- Do I have to live on the property?
- No. Atherton has no owner-occupancy rule for ADUs permitted on or after January 1, 2020, and state law (AB 976) now bars those rules permanently.
- Can I sell the ADU separately?
- Not today. A city has to adopt its own ordinance under AB 1033 before ADUs can be sold separately as condominiums, and as of January 2026 Atherton had not.
- What will ADU permits cost in Atherton?
- The San Mateo County ADU Resource Center lists about $1,380 in planning fees and $5,600 in building fees for a 1,199 square foot Atherton ADU, and the Town has waived several permit fees. Sewer, water and fire district fees are separate. ADUs of 750 square feet or less pay no impact fees.
- Is there a pre-approved plan option in Atherton?
- Yes. Atherton runs a pre-approved ADU process. Your lot still needs a geotechnical report, tree protection and Menlo Fire sign-off, so it saves design time more than review time.
- Can I legalize an older guest house built without permits?
- Often, yes, if it was built before 2020. State law (AB 2533) bars impact fees and penalties for those units, though the Town can deny one that is substantially unsafe. We can assess what it would take to bring yours up to code.
- What is the difference between an ADU and a junior ADU?
- A junior ADU is up to 500 square feet and sits inside the walls of the main house. It needs its own efficiency kitchen but can share a bathroom with the house. An ADU can be attached or detached, has its own full kitchen and bathroom, and can be much larger. A lot can have both.
- What is the smallest ADU I can build?
- An efficiency unit with at least 190 square feet of living space, under the 2025 California Building Code as cited in the state's ADU handbook. That is why a small studio or pool house can often become an ADU.
- How long does PG&E take to connect an ADU in Atherton?
- PG&E reported ADU electric hookups averaging about 300 days from request for projects finished by August 2025, and says its times have improved since. We file with PG&E when the permit goes in so power is not the last thing we wait on.
- Is the California $40,000 ADU grant still available?
- No. CalHFA's ADU Grant paid up to $40,000 toward predevelopment costs, and CalHFA says its latest round was fully allocated in December 2023. CalHFA warns that anyone offering to get you one is running a scam.
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