Portola Valley · San Mateo County
Custom Homes, Guest Houses & Estate ADUs in Portola Valley, California
Slopes, septic, and tree ordinances. Portola Valley is exactly where a modular ADU shaves months off a conventional build.
Building custom homes and estate ADUs in Portola Valley, California
SHAPE designs and builds custom homes, guest houses, and estate ADUs in Portola Valley and nearby communities, including Westridge, Woodside Highlands, Ladera, Los Trancos Woods, and the Alpine Road corridor. Ladera and Los Trancos Woods are unincorporated San Mateo County communities, so their permit jurisdiction differs from the Town of Portola Valley.
Portola Valley is rural, wooded, and slow-growth by design. A well-sited ADU here is usually a guest suite or a private studio, quiet, energy-efficient, and built to blend into the landscape.
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 Portola Valley project moves without the usual finger-pointing between architect, contractor, and city.
Portola Valley Building Standards at a Glance
The ADU standards below apply to accessory dwelling units. A new primary residence or a different type of accessory building follows its own requirements.
- Setbacks: California state law caps rear and side setbacks for new ADUs at 4 feet, and Portola Valley follows that standard.
- Height: The allowed height depends on the jurisdiction's standards, the approval path, and whether the ADU is attached or detached. State-law height exceptions have specific conditions; confirm the applicable limit before setting the roofline.
- Maximum size: Portola Valley publishes options up to 800 square feet under the state-exempt path and up to 1,200 square feet under its local ADU ordinance. Geologic conditions, fire access, and utility capacity require a site-specific review.
- 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.
- Number of units: Up to three ADUs and junior ADUs combined on a property.
- Front setback: Follows zoning, including Scenic Corridor setbacks.
- Parking: Portola Valley's published ADU requirements do not require parking for the ADU.
Rules checked September 2026 against: Town of Portola Valley ADU quick overview, San Mateo County ADU Resource Center, HCD letter to Portola Valley, Aug 2023, Town fire hazard maps, The Almanac on Portola Valley insurance, Nov 2024, 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 Portola Valley planning department, zoning verification, building plans, Title 24 energy, and every correction cycle, so you don't have to learn the code.
Read the Portola Valley ADU planning notes for the property checks behind these rules.
Our Process
- Site assessment. We walk your Portola Valley 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 Portola Valley.
- Build. Modules are fabricated in our facility while site work, foundation, utilities, landscaping, runs in parallel. Set and finish happen on site.
Planning an ADU on a Portola Valley property
Confirm the jurisdiction before applying Town standards
A Portola Valley postal address and the Town boundary are not the same thing. Nearby Ladera and Los Trancos Woods are unincorporated San Mateo County communities. Start with the parcel's jurisdiction before using a Town checklist.
For property within the Town, the ADU requirements distinguish the state-exempt option from the local-ordinance option. Select the path together with the proposed footprint and intended use.
A family ADU and a new custom home also have different review requirements. If both are planned, identify the shared site work without assuming the same approval route or schedule for both buildings.
Establish wastewater capacity before adding another bedroom
The Town specifically highlights sewer or septic capacity when an ADU adds bedrooms. A connection that appears short on a site plan may still require approval or system work.
For septic service, coordinate the applicable review with San Mateo County Environmental Health. For sewer service, confirm the serving district and its requirements; the Town identifies West Bay Sanitary District in its guidance. These outside-agency steps need to be accounted for alongside the Town permit.
Before choosing between one and two bedrooms, ask what the existing system can support and what an expansion would involve. That makes the layout decision more informed than comparing the room sizes alone.
Two numbers help here. San Mateo County sizes septic systems at 150 gallons a day per bedroom with a full reserve area. And West Bay Sanitary District has moved to bill ADUs it was never told about, including back charges in Portola Valley, so we register the ADU with the district from the start.
Review geology, fire access, and the route between buildings together
The Town identifies geologic and ground-movement maps as relevant to certain parcels, along with fire-access considerations such as road width, slope, and ingress or egress. These are site-specific conditions, not a reason to assume every Portola Valley property has the same limitation.
The Town's overview of the building process explains the involvement of planning, building, engineering, geologic, and fire review.
For a sloped location, put the proposed foundation, retaining work, drainage route, and daily pedestrian route on coordinated drawings. A location that gives the living room a better view may also create a longer or steeper route to the main house. Resolve that tradeoff in the family brief.
The Town adopted its geologic and ground-movement maps as policy in 2017. On the fire side, the Woodside Fire Protection District has required 100 feet of defensible space since May 30, 2024.
Compare attached and detached utility scopes
Portola Valley's published requirements distinguish the electrical connection for an attached or internal unit from a detached ADU supplied from the main house. The latter requires the private connection from the house to be underground.
That difference can affect trenching, tree coordination, and landscape restoration. Compare the complete service route when weighing a detached suite against an attached option.
Ask for the budget to identify foundation work, wastewater work, electrical connections, access, drainage, and restoration as well as the building. Any geotechnical recommendations should be reflected in the scope being priced. This makes it easier to understand which costs follow from the selected location and which follow from the interior design.
Request a Portola Valley site assessment with the address, bedroom count, and any survey, septic, or geotechnical records you already have.
Guest house or ADU in Portola Valley?
A separate building without a kitchen is an accessory building in Portola Valley. At 400 square feet or more, or two stories, it goes to the Architecture and Site Control Commission. An ADU with a kitchen is approved by staff without that review.
| Guest house without a kitchen | ADU with a kitchen | |
|---|---|---|
| Review | ASCC review at 400 sq ft or two stories | Staff approval, no ASCC review |
| Floor area | Counts toward the lot's floor area. No single building may use more than 85% of the maximum. | State-law ADUs do not count toward floor area. Others do. |
| Size | Set by the lot's floor area | Up to 1,200 sq ft |
In Portola Valley, an ADU is usually the faster path to a guest house.
Costs beyond the building on a Portola Valley lot
On a Portola Valley lot, wastewater and utilities add costs that come from the county, the sanitary district and PG&E rather than the builder. We price the ones your lot needs before you sign.
| Item | Amount |
|---|---|
| Septic site evaluation and perc test (San Mateo County, 2026) | $3,936 |
| County septic review with the building permit | $1,968 |
| New septic system permit | $3,773 to $6,559 |
| 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 |
Sources: San Mateo County Environmental Health fees, West Bay Sanitary District, PG&E ADU service report
Timeline for a Portola Valley ADU
In Portola Valley, geologic review and sewer or septic sign-off run alongside the Town permit. Here is how the main 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 |
| 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 |
Where a parcel needs geologic review, we start it with the application so it does not hold up the rest.
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 Portola Valley?
- Custom homes and estate-caliber ADUs in Portola Valley 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 Portola Valley?
- Portola Valley'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?
- Portola Valley custom homes follow the local off-street parking standards. Portola Valley's published ADU requirements do not require parking for the ADU.
- Can I build a guest house and an ADU on the same lot?
- The answer depends on the permitted use of each building, the existing structures, and the site's development limits. A guest house without independent living facilities is not automatically an ADU. Confirm the allowed combination and any conversion requirements before designing both buildings.
- 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 Portola Valley 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. Where a parcel has geologic or fire-access limits, those shape where the ADU can go rather than whether you can build one.
- Are fire sprinklers required in a Portola Valley ADU?
- Only if the main house is required to have them. State law sets that rule, and in August 2023 the state housing department told Portola Valley that a local ADU sprinkler mandate conflicted with it.
- Does a Portola Valley 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. An attached ADU counts as an addition and does not need solar.
- 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, so we price that in.
- Will an ADU raise my property taxes in Portola Valley?
- 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. State law (AB 976) permanently bars towns from requiring the owner to live on a property with an ADU.
- 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 Portola Valley had not.
- What do ADU permits cost in Portola Valley?
- A 2022 sample from the San Mateo County ADU Resource Center showed about $11,800 in Town fees for an 800 square foot ADU, including geologic review. ADUs of 750 square feet or less pay no impact fees under state law. Sewer or septic costs are separate (see the table above).
- Can I legalize an older unpermitted unit in Portola Valley?
- Often, yes. State law (AB 2533) covers ADUs built without permits before 2020. The Town cannot charge impact fees or penalties for them, but it can deny a unit 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.
- Is my Portola Valley parcel in a fire hazard zone?
- Check the CAL FIRE map the Town posted after its February 24, 2025 release. The Almanac reported that parts of Portola Valley fall in high or moderate zones on the 2025 local map. In a designated zone, a new ADU follows the 2025 Wildland-Urban Interface Code, which sets roof, vent, window and deck requirements.
- Will an ADU change my home insurance?
- Tell your insurer before the ADU is occupied, because it adds value to cover. In fire hazard areas, California's Safer from Wildfires program requires insurers to offer discounts for measures such as a Class A roof and ember-resistant vents, and a new ADU built to the 2025 Wildland-Urban Interface Code already has both. In Portola Valley, officials reported in November 2024 that 7 of the 12 largest insurers had paused or restricted new policies since 2022, and that Firewise community membership brings about a 10% discount.
- 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 Portola Valley?
- 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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