The maximum size of a home you can build in Los Altos, California, is determined by the city’s zoning regulations, specifically the floor area ratio (FAR) for residential districts. Los Altos is primarily zoned for single-family homes under districts like R1-10 (minimum lot size 10,000 sq ft), R1-20 (minimum 20,000 sq ft), and R1-40 (minimum 40,000 sq ft). The maximum allowable floor area varies based on your lot’s net area (gross lot size minus any easements or rights-of-way) and the specific zoning district, but the calculation is consistent across these R1 districts.
Key Calculation for Maximum Floor Area
- For lots with a net area of 11,000 sq ft or less: Maximum floor area = 35% of the net lot area.
- Example: On a 10,000 sq ft lot, the max is 3,500 sq ft.
- For lots with a net area greater than 11,000 sq ft: Maximum floor area = 3,850 sq ft + 10% of (net lot area - 11,000 sq ft).
- Example: On a 15,000 sq ft lot, the max is 3,850 + 0.1 × (15,000 - 11,000) = 4,250 sq ft.
- Example: On a 20,000 sq ft lot, the max is 3,850 + 0.1 × (20,000 - 11,000) = 4,750 sq ft.
This represents the total gross floor area, measured to the outside of exterior walls and including areas like garages, hallways, and habitable spaces across all levels (definitions are in Chapter 14.02 of the Los Altos Municipal Code). Fully below-grade basements are typically exempt from FAR calculations if they don’t extend beyond the footprint of the floor above and remain unexposed (no contour grading to make them appear as an additional story), allowing you to add significant extra space underground. Partially above-grade basements, attics with certain headroom, enclosed garages, and covered porches may count fully or partially—confirm specifics with the city.
Other Key Restrictions
- Lot Coverage: This limits the ground-level building footprint (measured under the roof in plan view) to preserve open space. While not as strictly defined as FAR in all districts, structures over 6 feet high are limited to 35% coverage in the required rear yard setback area. Overall lot coverage is often effectively capped around 35-40% due to setbacks and other rules, but it’s calculated separately from FAR.
- Setbacks and Height: Homes must comply with minimum setbacks (e.g., 20-25 ft front, 10-20 ft sides/rear in R1-10), maximum height (typically 30 ft), and daylight plane requirements to avoid shading neighbors.
- Exceptions and Bonuses: ADUs (accessory dwelling units) may be partially or fully exempt from FAR depending on size and type. There could be bonuses for sustainable features (e.g., green building elements), but variances are rare and require approval. Hillside lots or historic properties may have stricter limits.
- Site-Specific Factors: Slope, trees, easements, flood zones, or neighborhood design guidelines can reduce the buildable area further.
These rules are designed to prevent oversized “McMansion” developments and maintain neighborhood character. The examples above are for standard flat lots; actual limits can vary. For your specific property, check the zoning via the Los Altos zoning map and consult the Planning Division at City Hall (650-947-2750 or [email protected]) or a local architect—they’ll calculate based on your lot survey. Building permits require plan review, and non-conforming existing homes may limit expansions. 24 21