Picture two lots that hit the market in San Carlos the same week. One sits in Howard Park, flat and walkable, 5,200 square feet a few blocks off Laurel Street. The other is up in Cordilleras Heights, a hillside parcel of 9,000 square feet with mature oaks and a peek of the Bay. The Cordilleras Heights lot costs more to buy, and it costs more per square foot too. Under the city's own zoning formula, it also has a lower maximum floor area as a share of its lot.
That is not a typo, and it is not a quirk of one particular street. It is how San Carlos's Single-Family House Size Study works, an ordinance the City Council adopted on November 13, 2018 after two years of neighborhood meetings, a resident advisory committee, and public testimony from people worried that new houses were getting too big for their lots. The study sets maximum floor area according to lot size, using a formula for lots at or below 7,500 square feet and a flat 50 percent cap for larger lots. Anyone comparing a flatland fixer to a hillside estate in this city needs to understand how the formula applies before running the numbers on a remodel or rebuild.
The Formula Behind Every Remodel Permit
San Carlos calculates maximum floor area with two different rules depending on lot size, and the split happens at 7,500 square feet. For a lot at or under that size, the maximum is whichever number is larger: 1,100 square feet plus 35 percent of the lot area, or a flat 50 percent of the lot area. For anything above 7,500 square feet, there is no choice involved. The maximum is a flat 50 percent, period.
That flat-50-percent rule sounds simple enough that most buyers assume it applies uniformly across the city. It does not. The small-lot formula can set maximum floor area above 50 percent of the lot area on modest parcels; that share approaches 50 percent as lot size increases.
Running the ordinance's own formula across a range of lot sizes makes the pattern visible:
| Lot Size | Maximum Floor Area | Maximum Floor Area as % of Lot Area |
|---|---|---|
| 5,000 sq ft | 2,850 sq ft | 57% |
| 6,000 sq ft | 3,200 sq ft | 53% |
| 7,000 sq ft | 3,550 sq ft | 51% |
| 7,500 sq ft | 3,750 sq ft | 50% |
| 8,000 sq ft | 4,000 sq ft | 50% |
| 10,000 sq ft | 5,000 sq ft | 50% |
| 12,000 sq ft | 6,000 sq ft | 50% |
These figures come straight from the city's own Single-Family House Size Study formula and do not account for site-specific exemptions or the Residential Design Review Committee's separate review of projects that cross certain thresholds. A San Carlos buyer or their contractor should always confirm the applicable number with the Planning Division before finalizing renovation plans.
How the Maximum Floor-Area Share Changes with Lot Size
The math above answers a question most buyers never think to ask: why does a bigger lot not simply buy a proportionally bigger house allowance? The answer is that the two formulas cross paths at roughly 7,333 square feet. Below that point, the small-lot formula's fixed 1,100 square feet means maximum floor area exceeds 50 percent of lot area. From roughly 7,333 through 7,500 square feet, the 50 percent figure is the larger of the two calculations. For lots larger than 7,500 square feet, the flat 50 percent rule applies, no matter how much larger the lot gets from there.
In practice, the formula sets the maximum floor area for a 5,000-square-foot lot at 57 percent of its lot area, while a 12,000-square-foot hillside lot has a maximum floor area equal to 50 percent of its lot area. The extra land in Cordilleras Heights buys view, privacy, and yard. It does not buy proportionally more house.
Sloped Terrain Adds Another Consideration
The math cap is only one consideration for a hillside buyer. San Carlos's larger, elevated lots concentrated in Cordilleras Heights and the western foothills sit on sloped terrain, which is another factor to account for when evaluating a property.
Put the two considerations together and the picture sharpens. The hillside lot that costs more to buy, per square foot, has a maximum floor-area share of 50 percent under the ordinance. Both details are worth accounting for when comparing properties.
San Carlos's sub-neighborhoods illustrate how directly this connects to lot size and terrain:
- White Oaks and Howard Park sit close to downtown on smaller, flatter parcels, generally in the range where the small-lot formula can set maximum floor area above 50 percent of lot area.
- Eaton runs roughly 5,000 to 7,000 square feet, a post-war flatland pocket where the small-lot formula can set maximum floor area above 50 percent of lot area.
- Beverly Terrace typically runs 6,000 to 8,500 square feet, straddling the crossover point depending on the individual parcel.
- Cordilleras Heights, at higher elevation with lots commonly between 7,500 and 12,000 square feet, is where the flat 50 percent rule applies.
- Oak Park rounds out the city's older, walkable pockets with lot sizes closer to the small-lot end of the scale.
What This Means for Anyone Underwriting a Remodel or Rebuild
San Carlos is not a market where floor-area math is a minor detail. Redfin's data for the three months ending July 2026 put the city's median sale price at $2.5 million, with homes selling in about 12 days. Local single-family closings tracked through the San Mateo County Association of Realtors and MLSListings showed a July 2026 median of roughly $2.7 million, with homes averaging 22 days on market and closing at 109 percent of list price. Land in this city is expensive by any measure, and inventory has stayed tight enough that competitive offers are still the norm rather than the exception.
When every square foot of lot costs this much, the percentage of that lot you are legally allowed to build on stops being a paperwork detail and becomes part of the underwriting. A buyer comparing a smaller in-town lot against a larger hillside one should run both scenarios: the smaller lot's floor-area formula alongside its purchase price, and the larger lot's flat 50 percent cap alongside its price and view. The lot that looks like more house on paper is not always the lot that delivers more usable floor area once the ordinance is factored in.
One detail worth building into any renovation plan either way: up to 250 square feet of unenclosed, unroofed decks aligned with or below the lowest floor level are exempt from the lot coverage calculation entirely. That can be useful when planning outdoor space, though the exemption applies to lot coverage, not the maximum floor-area calculation.
A Few Questions We Get From Buyers
How does the formula differ for lots above 7,500 square feet? Lots above 7,500 square feet use a flat 50 percent maximum. Lots at or under 7,500 square feet use whichever is larger: 50 percent of the lot area or 1,100 square feet plus 35 percent of the lot area.
Are decks exempt from the lot coverage calculation? Up to 250 square feet of unenclosed, unroofed decks at or below the lowest floor level are specifically exempted from the city's lot coverage calculation, giving owners some flexibility when planning outdoor space.
If you are comparing a flatland fixer against a hillside lot in San Carlos, or trying to figure out what a given parcel actually lets you build, the math is worth running before you write an offer, not after. The Canlas Brothers work through this kind of lot-by-lot underwriting with buyers and investors across San Carlos every week. Schedule your free Home Strategy Consultation and we will walk your specific address through the formula together.
This article is general information, not legal, tax or financial advice. Consult a qualified professional about your situation.
