Basements in Bay Area Cities: Do They Count Toward Maximum Floor Area?
Basements remain a smart way for homeowners in Silicon Valley to gain extra space—whether for family rooms, offices, gyms, or storage—while often staying within zoning limits on above-ground bulk. The key factor is whether basements are included in floor area ratio (FAR) or total allowable floor area calculations, which vary by city and depend on how “below grade” the space is, exposure of walls, lightwells, and other details.
Below is an updated overview for Los Altos, Los Altos Hills, Mountain View, Palo Alto, Woodside, Portola Valley, and Cupertino (focusing on single-family residential zones like R1). Rules are drawn from municipal codes and planning resources as of 2026. These can change, and site-specific conditions (slope, geology, flood zones) matter—always confirm with your city’s planning department.
Los Altos
Fully below-grade basements are typically exempt from FAR if unexposed and within the building footprint. Partially exposed or “daylight” basements usually count fully.
Los Altos Hills
Basements are exempted from floor area if the floor above is ≤28 inches above grade (natural or finished, whichever lower) and ≥75% of the perimeter is underground. Bunkers >1,500 sq ft count as floor area; detached or non-footprint basements may count as development area unless under counted surface.
Mountain View
Fully below-grade basements are generally exempt from FAR (standard practice; basements don’t count as stories in height calcs for ADUs). Confirm with Planning, as accessory structures count toward permitted floor area.
Palo Alto
Basements are excluded from FAR if the first floor is no more than 3 feet above grade and they comply with lightwell/excavation rules. Habitable basements can still be exempt under these conditions; small exposed areas are allowed without counting.
Woodside
Basements are typically exempt from total floor area (TFA) calculations. Garages within basements may also qualify for exclusions, though large grading/exposed basements require review.
Portola Valley
Basements are partially exempt: Up to 20% of the above-ground floor area doesn’t count toward adjusted maximum floor area (AMFA). Larger basements reduce allowable above-ground space (e.g., trading 2 sq ft basement for 1 sq ft house).
Cupertino
Basements are exempt from floor area if they include conforming lightwells (per code Section 19.28.070). This applies to single-family homes; ADUs ≤800 sq ft are also exempt from FAR overall.
Comparison Chart: Basements and FAR Across Cities
|
City |
Counted in FAR? |
Key Conditions/Exemptions |
|
Los Altos |
No |
Fully below grade, unexposed, within footprint |
|
Los Altos Hills |
No |
≤28” above grade, ≥75% underground; >1,500 sq ft bunkers count |
|
Mountain View |
No |
Generally exempt if fully below grade (confirm with city) |
|
Palo Alto |
No |
First floor ≤3’ above grade; lightwell compliance; small exposed OK |
|
Woodside |
No |
Exempt; special review for large/exposed |
|
Portola Valley |
Partial |
Exempt if ≤20% of above-ground area; larger count toward AMFA |
|
Cupertino |
No |
Exempt with conforming lightwells |
Final Tips
- Basements add value — Often 50-70% ROI in the Bay Area, plus energy efficiency.
- Next steps — Get a site survey, consult an architect, and submit plans early. Many cities allow exemptions for sustainable features.
- Why the variation? Rules balance growth, aesthetics, and environmental concerns in hilly or dense areas.
Planning a basement? Check your city’s official planning portal or call the department for the latest details. Let me know if you need help with a specific lot or more visuals! This information is provided as a courtesy but should not be relied on - interested parties should got the specific city with your specific project information to confirm.