We start with your design. Modular is how we build it, not what you settle for.
Shape Homes · Where we build · Portola Valley
Two things make Portola Valley different from every town around it. Whether your project is ministerial turns on a percentage, not a size. And the Town has its own geologist.
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| Rule | What it says | Source |
|---|---|---|
| ADU, state-exempt size | 800 sq ft | Town ADU quick overview |
| ADU, local ordinance maximum | 1,200 sq ft | Town ADU quick overview |
| JADU | 500 sq ft | Town ADU quick overview |
| Height, detached up to 800 sq ft | 16 ft | Town ADU quick overview |
| Setbacks, rear and side | 4 ft minimum | Town ADU quick overview |
| Setback, front | Underlying zoning, including Scenic Corridor setbacks | Town ADU quick overview |
| Parking | None required | Town ADU quick overview |
| Decision timeline | 60 days from a complete ministerial application | Town ADU quick overview |
| Fire sprinklers | Typically required for newly constructed ADUs | Town ADU FAQ |
| Full ordinance text | PVMC 18.36.040 — confirm current text with the Town | PVMC 18.36.040 |
Sources: Town of Portola Valley ADU quick overview, Town overview of building an ADU, San Mateo County ADU Resource Center, HCD letter to Portola Valley, Aug 2023, Town fire hazard and geologic maps. Rules change often; we confirm them for your lot before design.
Size is rarely the constraint. These are.
Adjusted Maximum Floor Area is the governing local cap. An attached ADU that pushes the main residence past 85% of AMFA triggers discretionary review. So does an internal ADU over 1,200 sq ft, and so does a parcel adjacent to a Scenic Corridor. Two identical units on two identical lots can take completely different routes through the Town. We run the AMFA number first.
Portola Valley states plainly that certain parcels may be restricted with respect to proximity to earthquake faults and traces, and areas subject to active downslope movement. Applications route to the Town Geologist. This is not a formality on this side of the range, and it is the reason a lot check here starts with maps rather than a tape measure.
A Portola Valley application passes Building, Planning, Engineering, the Town Geologist and Woodside Fire Protection District. Where the parcel is on septic, capacity to serve the new unit has to be demonstrated to San Mateo County Environmental Health. Sequencing those five is most of the schedule.
A Portola Valley postal address does not always mean the property is governed by the Town. Ladera and Los Trancos Woods include unincorporated San Mateo County properties. We establish the parcel's jurisdiction before applying an ADU standard or naming the reviewing department. Westridge, Woodside Highlands and Alpine Road properties also bring different terrain and service conditions, so the location check comes before choosing the plan.
The property may be served by sewer or septic, with different approval and connection requirements. We identify the provider or environmental-health authority, review existing records and confirm capacity for the intended use. An additional bedroom can change the wastewater scope on a septic lot. That decision belongs in feasibility alongside the building location, reserve areas and the route of the underground services.
Portola Valley's geologic and ground-movement information helps screen a proposed location, followed by the parcel-specific review required for the work. We test that location against fire access, driveway conditions and the route between buildings. A site section makes the floor level, foundation, retaining work and drainage understandable. Where specialist review is needed, it is scheduled before committing to a footprint that may need to move.
An attached option and a detached ADU can serve the same family brief with different utility, foundation and access scopes. We compare them on the surveyed property, including underground service extensions, wastewater work and any landscape restoration. The selected approval path and floor-area calculations are checked at the same time. A useful estimate shows those site differences clearly alongside the rooms and finish package.
The property checks behind the standards above.
San Mateo County sizes septic systems at 150 gallons a day per bedroom with a full reserve area. Before choosing between one and two bedrooms, we ask what the existing system can support and what an expansion would involve, so the room count is an informed decision rather than a guess at the floor plan stage.
West Bay Sanitary District has moved to bill ADUs it was never told about, including back charges on Portola Valley properties. We register the ADU with the district as part of the permit sequence instead of leaving it for the district to discover later.
The Town adopted its geologic and ground-movement maps as policy in 2017, and certain parcels are restricted with respect to proximity to earthquake faults and traces or areas subject to active downslope movement. On the fire side, the Woodside Fire Protection District has required 100 feet of defensible space since May 30, 2024. Both get checked before the footprint is set.
Portola Valley's published requirements distinguish the electrical connection for an attached or internal unit from a detached ADU, which must be supplied from the main home through an underground private connection. That difference affects trenching, tree coordination and landscape restoration, so we compare the full service route before choosing attached over detached.
Wastewater and utility costs are set by the county, the sanitary district and PG&E, not by us. 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.
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 | A few months; we confirm the schedule in your agreement |
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.
Three from the library, chosen against this town's rules rather than its postcode.
Planning ranges, all in: factory package, delivery, foundation, utilities, permits and site work.
What moves a project inside its range is the lot, not the home. On Portola Valley parcels the items above are what usually decide it. A range is not a bid; a fixed price applies only as written in an executed construction agreement. See the plans or how the build works.
Most of our Portola Valley projects are called guest houses by the people who own them. A few are called pool houses, and one or two are called in-law cottages. Legally they are the same thing: an accessory dwelling unit. There is no separate guest house permit in Portola Valley. The rules described on this page, the size caps, the setbacks, the review path, apply to all of them, whatever the drawings call the building. That is good news. The ADU path is the fastest approval route available on a Portola Valley lot, so a pool house or in-law cottage designed as an ADU clears review sooner than the same building filed any other way.
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.
Adjusted Maximum Floor Area is Portola Valley's local floor-area cap. It matters because ministerial versus discretionary review turns on it. An attached ADU that takes the main residence past 85% of AMFA goes discretionary. That is a different timeline and a different risk profile from a by-right approval, and it is decided by arithmetic you can do before you draw anything.
Up to 800 sq ft is state-exempt. Up to 1,200 sq ft is allowed under the local ordinance. A junior ADU is capped at 500 sq ft.
Portola Valley's front setback for an ADU follows the underlying zoning, including any Scenic Corridor overlay. On a corridor parcel that pushes the front setback well beyond the base zone. It is one of the few places where the front line, not the rear, decides where a second unit can go.
Typically yes for newly constructed ADUs. In a Very High Fire Severity Zone, conversions are workable but new construction requires a Planning Department check on street access, slope and road width before anything else.
Yes. In Portola Valley a guest house, pool house or in-law cottage is permitted as an accessory dwelling unit, so the size caps, setbacks and review steps described above apply exactly as written. The label on the drawings does not change the path. What changes the answer is the lot: its size, its slope, and what is already built on it.
No. Some nearby communities, including Ladera and Los Trancos Woods, contain unincorporated County properties. We confirm the parcel jurisdiction and responsible planning, building and environmental-health authorities before applying the ADU requirements.
Unresolved wastewater capacity, geologic review, fire access or utility approvals can hold up the project even when the room layout is settled. We identify those dependencies during feasibility and coordinate the required reviews with the permit package.
It is often useful where terrain or long service routes make a detached location more involved. We compare privacy, daily access, foundation work, utilities and the applicable approval provisions for both options, using the same family brief and surveyed property.
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.
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.
Tell your insurer before the ADU is occupied, because it adds value to cover. California's Safer from Wildfires program requires insurers to offer discounts for measures such as a Class A roof and ember-resistant vents. 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.
We pull your parcel, the Portola Valley ordinance and a street view of the driveway before the call.
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