Portola Valley — Overlay Districts

Last reviewed: July 2, 2026

Overview

Portola Valley’s zoning title uses a mix of combining districts and special overlay rules to add site‑specific regulatory layers on top of base zones. These include combining districts (commonly called “combining” or “overlay” districts) such as D-R (Design Review), F-P (Floodplain), slope‑density (S‑D), and area‑specific overlays like the Affiliated Housing (AH) combining district and special setback lines S. The controlling rules are in Title 18 of the Portola Valley Municipal Code; read important definitions and map applicability before you assume a parcel is unaffected. See the town’s core zoning overview at Portola Valley Zoning for context.

NOTE: This page is strictly an interpretation of Portola Valley’s zoning ordinance materials supplied; parcel‑specific situations must be confirmed with the town. Where I could not find a specific list of uses or numeric standard in the retrieved materials I state "Not found in retrieved materials" and indicate where to verify.

What Portola Valley calls “overlays”

Portola Valley organizes its overlays in two ways:

• As formal combining districts (listed on the zoning map and adopted in Title 18), e.g., D-R, F-P, S‑D, AH, etc. See § 18.06.020 for the combining district list and map mechanics.
• As specialized regulatory chapters that act like overlays (for example, special setback lines S in Chapter 18.58 and floodplain mapping in Chapter 18.32) — when adopted they are shown on the zoning map and supersede ordinary yard rules where specified.

Below is a district‑by‑district breakdown of the most commonly applied Portola Valley overlays / combining districts found in the code excerpts you provided.

D-R (Design Review) combining district

• Purpose: Preserve visual and natural character, adapt development to scenery, and protect steep/erosion/fire‑prone areas; established to protect major circulation corridors as well. See § 18.30.010.
• Typical permitted uses: Uses remain those of the underlying zoning district; the D‑R layer triggers Architectural and Site Control Commission (ASCC) review for form, materials, color, and planting (building permit applications are subject to ASCC approval). See § 18.30.030.
• Key dimensional / process effects: ASCC may adopt administrative rules and waivers; tree species for planting in D‑R must come from the conservation committee list and all building permit applications in D‑R require ASCC attention. See § 18.30.030.
• Where it applies: Locations are set on the zoning map; the chapter explicitly ties the D‑R boundaries to the map adopted under § 18.08.010. See § 18.30.020.

First time discussing design review? Expect to need design review materials and the ASCC process — consult Portola Valley Design Review for procedural details.

F-P (Floodplain) combining district

• Purpose: Regulate development in mapped special flood hazard areas to comply with FEMA FIRMs adopted by reference. See § 18.32.030.
• Typical permitted uses: Underlying uses are controlled, but development within mapped flood areas must meet Chapter 18.32 requirements; development permits are required before building in regulated areas (§ 18.32.035).
• Key dimensional / process effects: Flood rules modify how you measure adjusted parcel area for impervious surface calculations (see Table No. 1B and the AMIS computations in the code) and require flood‑elevation documentation for permits. See the flood and impervious surface computation tables.
• Where it applies: To parcels shown on the FEMA FIRMs incorporated into the zoning ordinance (map is on file at Town Hall). See § 18.32.030.

If your parcel touches a mapped flood area, expect additional submittals and engineering requirements; check the town maps and Chapter 18.32.

S (Special setback lines — scenic and hazard setbacks)

• Purpose: Special setback lines are overlaid to protect scenic corridors and to keep structures away from hazards (scenic corridors, earthquake faults, floodplains). See § 18.58.010 and § 18.58.020.
• Typical permitted uses: Underlying uses still permitted, but no structure can be located closer than the adopted special setback line to a right‑of‑way or hazard trace; the special setback supersedes ordinary yard requirements where it is more restrictive. See § 18.58.020(B).
• Key dimensional standards (examples adopted in code):
  • Skyline Boulevard: no closer than 200 ft from ROW (ASCC may reduce to 100 ft in special circumstances) — § 18.58.020.D.1.
  • Alpine Road (specific section): 75 ft from ROW — § 18.58.020.D.2.
  • Portola Road (specific segments): 50 ft for 1‑acre+ parcels; different numeric setbacks for smaller parcels and one‑story vs. two‑story houses (see § 18.58.020.D.3).
• Where it applies: The special setback lines are adopted and shown on the zoning map per Chapter 18.74 procedures and the map in § 18.08.010.

Because the code lists multiple, location‑specific numeric setbacks, always confirm which segment of a corridor applies to a parcel and whether ASCC discretion could alter the setback. See the town’s Development Standards and Special Setback rules.

AH (Affiliated Housing) combining district

• Purpose: Incentivize employer/institution‑sponsored workforce housing and provide site flexibility while keeping compatibility with the base district (objective: encourage employees of affiliate institutions to live locally). See § 18.29.010.
• Applicability: Only applies to parcels identified on the zoning map as AH; any owner of an employment or institutional parcel may apply for AH designation in accordance with § 18.29.030. See § 18.29.020–030.
• Typical permitted uses & approvals: AH projects require a conditional use permit (Chapter 18.72) and ASCC plus Planning Commission review; housing must be offered first to employees of the affiliate institution (see § 18.29.040–050).
• Key dimensional / development standards: Density is parcel‑specific: example sites are listed (Sequoias up to 23 units, Christ Church up to 6 units), and development standards default to R‑MF standards depending on density (for ≤6 du/ac use R‑MF‑4 standards; for >6 du/ac use R‑MF‑23) per § 18.29.060. The planning commission may approve parking reductions with a conditional use permit.

If you’re an employer or institution exploring workforce housing, AH is the overlay to study first; it explicitly ties outcomes (density, design) to the Planning Commission and ASCC review processes.

P‑C (Planned Community) district

• Purpose: Allows unified, plan‑based development for large parcels (60+ acres), integrating housing with parks, open space, and facilities via an approved general development plan. See the P‑C chapter and § 18.28.070.
• Typical permitted uses: The approved P‑C plan controls uses; typical inclusions are R‑1/R‑E district principal uses plus horticulture/grazing and PUDs subject to conditional use permits. See § 18.28.050–070.
• Key standards: P‑C requires an approved general development plan and may be combined with S‑D slope density districts; once a P‑C is established development must conform to the plan. See § 18.28.130–150 and § 18.28.080 on S‑D combination.

P‑C is rarely applied; it’s a full plan overlay rather than a minor technical adjustment.

S‑D (Slope Density) combining districts and other combining districts

• Purpose / effect: Slope‑density combining districts (and the list of residential density combining districts) control parcel size, required open space, and density based on slope and terrain; they are an overlay class listed in § 18.06.020. Examples include 7.5M, 15M, 1A, 2A, etc.
• Key standards: Used in calculating permitted units for PUDs and applying Table No. 1 / Table No. 2 impervious surface and area/open‑space rules. See Chapters 18.50 and the computation tables (Table 1B) for impervious surface adjustments.

Quick standards table (decision‑relevant snapshot)

How overlays interact with core development rules

• Overlays do not replace the base zoning uses and bulk rules; they layer on additional conditions. The ordinance states the more restrictive provision governs where there is conflict (§ 18.02.050).
• Many overlays explicitly require the underlying chapters’ accessory structure, yard, and parking rules to be followed in addition to overlay standards (example: O‑A cites Chapters 18.42, 18.48–18.60 for accessory, bulk, and parking). See § 18.26.010 for O‑A and related cross‑references.
• Conditional uses, CUP findings, and design review are the principal discretionary tools for approving projects in overlay areas — see the CUP review factors in Chapter 18.72.

Because overlays frequently alter setbacks, impervious surface calculations, and required approvals, also confirm any overlay mapping on the official zoning map (zoning map adopted by § 18.08.010) before assuming development rights.

First time encountering parking effects? The town’s parking standards are in the parking chapter; confirm required vehicle counts early in your design submittal. See Portola Valley Parking.

Practical guidance for applicants (plain English synthesis)

• Always start with the zoning map: identify the base zone and any combining district codes shown on the parcel (the ordinance requires map adoption via § 18.08.010 and label combining districts in § 18.06.020). Confirm whether your parcel lies inside D‑R, F‑P, S, AH, P‑C or any slope density band.
• If in D‑R, budget time and materials for ASCC review (form, color, plant list). See Portola Valley Design Review.
• If in F‑P or within special setbacks (S), expect additional geotechnical/flood/habitat studies and nonstandard setback measurements; flood/fema maps are incorporated by reference and mapped in § 18.32.030 and § 18.58.020.
• For AH projects you must apply for AH designation (if not already mapped) and run a CUP through the Planning Commission; AH project density and development standards default to R‑MF tables as specified in § 18.29.060.

If your project includes an ADU, remember state ADU law overlays local rules — consult Portola Valley ADUs and California Building Standards Code for building requirements.

Checklist

• Confirm base zoning and all combining/overlay districts on the official Portola Valley zoning map (verify with Town Planning). § 18.08.010; § 18.06.020.
• Review any applicable overlay chapter text: D‑R (Ch. 18.30), F‑P (Ch. 18.32), S setbacks (Ch. 18.58), AH (Ch. 18.29), P‑C (Ch. 18.28).
• Determine whether project requires Conditional Use Permit or ASCC review (Ch. 18.72 and Ch. 18.64). § 18.29.040; Ch. 18.72.
• Check special numeric standards that may override base yards or heights (e.g., Skyline 200 ft, Alpine 75 ft, Portola Road rules). § 18.58.020.
• Prepare flood/ground movement, impervious area, and slope analyses if parcel is in F‑P, Pf, or slope density bands (see Chapter 18.50, Table 1B).
• Coordinate with conservation committee on plant lists and with ASCC early if in D‑R. § 18.30.030.

Risks & Ambiguities

Plain‑English summary

Portola Valley uses a set of combining and special districts (overlays) to put additional rules on top of base zones — the overlays control things like design review, scenic and hazard setbacks, floodplain development, and targeted programs such as affiliated housing; the ordinance spells out where overlays appear on the zoning map and which chapters apply (verify parcel mapping and follow required ASCC/CUP processes). See the specific overlay chapters cited below for the exact rules and numeric standards.

Source References

• Title 18 — Zoning (Portola Valley) — combining district list § 18.06.020.
• Special districts list / O‑A, P‑C, D‑R, H‑R, F‑P — Chapter 18.24.
• O‑A (Open Area) district—purpose and cross references § 18.26.010.
• P‑C (Planned Community) district — uses and conditions Chapter 18.28.
• Affiliated Housing (AH) combining district — § 18.29.010–060.
• D‑R (Design Review) combining district — § 18.30.010–040.
• Special setback lines (S scenic and hazard) — § 18.58.010–030, including numeric setbacks for Skyline, Alpine, Portola Road.
• Floodplain mapping and applicability — § 18.32.030–035.
• Planned unit developments, slope density and impervious surface computation references — Chapters 18.50 and Table No. 1B.
• Conditional use permit review standards and factors — Chapter 18.72.

If you want, I can pull the exact permitted‑use lists for a particular combining district or extract the full numeric tables for a named parcel — tell me the parcel's APN or the district name you need and I will verify the exact code subsections that apply.

Retrieved passages

• Portola Valley Zoning Code (§ 2) High relevance show source text
• Portola Valley Zoning Code (§ 1) High relevance show source text
• Portola Valley Zoning Code (Chapter 18.72) High relevance show source text
• Portola Valley Zoning Code (Section 18.54.050.) High relevance show source text
• Portola Valley Zoning Code (§ 1) High relevance show source text
• Portola Valley Zoning Code (Section 18.52.070) High relevance show source text
• Portola Valley Zoning Code (§ 5) High relevance show source text
• Portola Valley Zoning Code (TITLE 18) Medium relevance show source text
• Portola Valley Zoning Code (§ 1) High relevance show source text
• Portola Valley Zoning Code (§ 15) High relevance show source text
• Portola Valley Zoning Code (§ 1) High relevance show source text
• Portola Valley Zoning Code (§ 1) High relevance show source text
• Portola Valley Zoning Code (Section 18.29.030.) Medium relevance show source text
• Portola Valley Zoning Code (Chapter 18.43) Medium relevance show source text
• Portola Valley Zoning Code (Section 11353.1) Medium relevance show source text
• Portola Valley Zoning Code (§ 1) Medium relevance show source text

Cited sections

• Title 18 — Zoning (Portola Valley) — combining district list **§ 18.06.020**. (Title 18)
• Special districts list / O‑A, P‑C, D‑R, H‑R, F‑P — Chapter **18.24**.
• O‑A (Open Area) district—purpose and cross references **§ 18.26.010**. (§ 18.26.010)
• P‑C (Planned Community) district — uses and conditions **Chapter 18.28**. (Chapter 18.28)
• Affiliated Housing (AH) combining district — **§ 18.29.010–060**. (§ 18.29.010)
• D‑R (Design Review) combining district — **§ 18.30.010–040**. (§ 18.30.010)
• Special setback lines (S scenic and hazard) — **§ 18.58.010–030**, including numeric setbacks for Skyline, Alpine, Portola Road. (§ 18.58.010)
• Floodplain mapping and applicability — **§ 18.32.030–035**. (§ 18.32.030)
• Planned unit developments, slope density and impervious surface computation references — Chapters **18.50** and Table No. 1B. fileciteturn1file19
• Conditional use permit review standards and factors — Chapter **18.72**.
• PortolaValley_ZoningCode.md

• Official Portola Valley code library