Atherton, Woodside and Portola Valley Neighborhoods by Their Lots: Lot Sizes, Flood Zones and Fire Zones

How big are the lots in Atherton, Woodside and Portola Valley, and how many sit in a flood or fire zone? For Atherton and Woodside we took the parcels with San Mateo County’s single-family use code, 1,500 square feet or more, inside each of our neighborhood guides and measured each lot from the county’s parcel outline. For Portola Valley we took the parcels the county assessor lists as a single-family home in the Town of Portola Valley’s parcel layer, with the lot size from its acreage; Ladera and Los Trancos Woods & Vista Verde, unincorporated county land next to the town, come from the county layer. We checked each lot’s center against FEMA’s flood map and each lot against CAL FIRE’s fire hazard severity zone maps. Parcel records downloaded October 2026: the San Mateo County parcel layer, whose use codes were last updated in June 2020, and county assessor records in the Town of Portola Valley parcel layer. There are no addresses or owner details here.

The short answer

• Atherton: across our six guides the median single-family lot is 0.99 acres, from 0.51 acres in Lloyden Park to 1.08 acres in West of the Alameda. None has its parcel center in a FEMA high-risk flood zone, and only two touch a fire hazard severity zone (Moderate, in West of the Alameda).
• Woodside: across our eight guides the median single-family lot is 1.26 acres, from 16,100 sq ft in Woodside Glens to 3.28 acres in Mountain Home Road & Portola Road (guides with at least 150 lots); Kings Mountain & Skyline, with 48 lots, has a median of 5.29 acres. 1,356 lots (64%) touch a fire hazard severity zone (51 Very High, 909 High and 396 Moderate); three have their parcel center in a FEMA high-risk flood zone.
• Portola Valley: across the town, the median lot under a single-family house is 1.02 acres, from 13,960 sq ft in Woodside Highlands & Hayfields to 2.58 acres in Westridge. 1,498 houses (98%) have part of the lot in a fire hazard severity zone (601 High and 897 Moderate); 18 have their parcel center in a FEMA high-risk flood zone.
• Next to the town (unincorporated): Ladera has 533 single-family lots with a median of 12,290 sq ft, 417 of them in a fire hazard zone; Los Trancos Woods & Vista Verde has 256 single-family lots with a median of 0.59 acres, all of them in a fire hazard zone (123 Very High).

Atherton

2,460 parcels with the county’s single-family use code inside our six Atherton guides; no lot falls in two guides. Largest median lot first; each name opens its guide.

Woodside

2,125 parcels with the county’s single-family use code inside our eight Woodside guides. The county layer has no town-limit line, so a lot inside a guide’s outline counts whether or not it is inside the town. Mountain Home Road & Portola Road and Skywood, Portola Hills & Skylonda share an edge: the one lot inside both outlines counts in both, and once in the total. Largest median lot first; each name opens its guide.

Portola Valley

1,526 parcels in the town’s parcel layer that the county assessor lists as a single-family home, with lot sizes from the acreage in those records. Largest median lot first; each name opens its guide.

The “All of Portola Valley” row also counts the 37 houses outside the neighborhoods in the table. Ladera and Los Trancos Woods & Vista Verde are unincorporated San Mateo County land next to the town, outside its parcel layer; their figures come from the county’s parcel layer:

Flood zones, neighborhood by neighborhood

Lots in FEMA high-risk zones inside each guide’s outline, most first: three in Woodside and 18 in Portola Valley. No Atherton guide has any, and neither do Ladera or Los Trancos Woods & Vista Verde.

What the flood zones mean

• Zone A: a 1%-a-year chance of flooding, where FEMA hasn’t set a base flood elevation.
• Zone AE: the same 1%-a-year risk, with a base flood elevation set.
• Zone X: moderate to minimal risk, outside the 1%-a-year floodplain.
• Insurance: if the house itself is in an A or V zone, lenders on federally backed mortgages require flood insurance; a flood zone determination for the address settles it. In zone X it’s optional.

Fire hazard zones, neighborhood by neighborhood

CAL FIRE maps how severe the wildfire hazard is across California, in three classes: Moderate, High and Very High. Lots inside each guide’s outline, by the highest zone that touches any part of the lot, grouped by town; most severe first. In Woodside, 1,356 of 2,125 lots (64%) touch a zone; in Portola Valley, 1,498 of 1,526 houses (98%). Holbrook-Palmer Park & Town Center, Lindenwood, Lloyden Park, Selby Lane and West Atherton have no lot in any zone.

What the fire zones mean

• Hazard, not risk. CAL FIRE’s Office of the State Fire Marshal says the maps describe hazard: the physical conditions, like vegetation, terrain, fire history, embers and typical fire weather, over 30 to 50 years. They don’t count home hardening, defensible space or fuel reduction, which change the risk to a particular home (OSFM).
• Two kinds of map. Inside city limits (Local Responsibility Areas) we used CAL FIRE’s 2025 map; where the state is responsible for wildfire protection (State Responsibility Areas), its State Responsibility Area map. Land outside the three classes is shown as non-wildland or unzoned (OSFM).
• What a zone can bring. Depending on the zone and the kind of area, stricter building standards for new construction and additions, defensible-space rules, and a line on the seller’s natural hazard disclosure. The town’s building department (or the county’s, on unincorporated land) can say which apply.

Reading it right

• Two kinds of count. In Atherton, Woodside, Ladera and Los Trancos Woods we counted parcels the county codes as single-family residences, 1,500 square feet or more; the codes date from June 2020, so a few newer houses may be missing or coded otherwise. In Portola Valley we counted parcels the county assessor lists as a single-family home, including those with an ADU. Townhomes and condos aren’t counted anywhere.
• Lot sizes are measured from the county’s parcel outlines, or in Portola Valley taken from the acreage in the town’s parcel records, so they can differ from a survey.
• We checked the center of each lot against FEMA’s National Flood Hazard Layer. A lot can straddle two zones, and maps change, so check the current FEMA map for the exact address.
• The flood zone covers the land, not the house. A house can sit above the base flood elevation even in a high-risk zone. An elevation certificate settles it and can lower the insurance price.
• The fire zone describes the land around a house, not how the house is built or how the lot is kept. CAL FIRE’s zone viewer shows any parcel, and a seller’s natural hazard disclosure report states the zones for that property.
• A median isn’t a verdict. It describes the lots, not which neighborhood is right for you.

Go deeper

Atherton, Woodside and Portola Valley Home Prices by Neighborhood compares five years of house sales guide by guide. The Most Expensive Streets in Menlo Park & Atherton and The Most Expensive Streets in Woodside & Portola Valley go street by street. Flood & Fire Zones by Neighborhood compares every guide we write, and the Atherton, Woodside and Portola Valley city guides link every neighborhood guide.

More on Atherton, Woodside and Portola Valley: home prices by neighborhood · the most expensive streets in Menlo Park and Atherton · the most expensive streets in Woodside and Portola Valley · Menlo Park vs. Atherton · Atherton city guide · Woodside city guide · Portola Valley city guide

Buying, selling or rebuilding in a flood or fire zone?

A Property Nerd can pull the zoning, the lot, the flood and fire zones, any elevation certificate and the recent sales on your street before you write an offer, set a price or plan a rebuild. Call (408) 373-1660 or (650) 383-8606, or book a strategy call. Commissions are negotiable, and we offer bundled services, so ask us what’s included.