Among the homes for sale Hillsborough, CA has right now, many look like projects waiting to happen. Before you plan one, walk the property with the town’s rules in mind. Hillsborough reviews almost everything visible from outside, protects most mature trees and limits how large a house can grow.
Why the rules run so deep
Hillsborough has about 11,000 residents and no commercial streets at all. Its founders set out in 1910 to keep the place rural, going as far as banning sidewalks and grid-pattern streets. That thinking lives on in the town’s development standards, and it reaches most corners of a property.
So let’s start at the road and work back.
At the street: gates, fences and the front yard
Fencing and landscaping both count as exterior changes, so both need design review. Town staff usually handle these smaller projects, which is quicker than a public hearing. Still, plan on an approval step before ordering a gate or replanting the frontage.
The outside of the house
This is where many new owners get caught. All exterior changes need design review, and the town’s list includes re-roofing and changes to colors or materials. Staff can sign off on minor work. Painting a house a new color is therefore not the private decision it would be elsewhere.
A larger change of style, such as turning a ranch into something more traditional, generally needs preliminary review by the Architecture and Design Review Board.
The trees between you and the view
Stand in the yard and look up. Since July 2023, the town’s tree rules have protected trees 18 inches or wider in diameter, measured at 4 feet 6 inches. The old threshold was 36 inches, so many more trees are now covered.
On a lot with a house, removing a protected tree takes design review and a permit. Penalties for doing it without one run $150 per inch of trunk or the tree’s appraised value, whichever is greater. A big oak can decide where an addition goes, or whether it goes anywhere at all.
The footprint: how much bigger can it get?
Floor area is generally limited to 25% of the net lot area for the first acre, plus 15% of anything beyond an acre. A half-acre lot of 21,780 sq ft comes to about 5,445 sq ft on paper. The town’s board cautions that owners shouldn’t count on reaching the maximum.
To see how that plays out, look at Hillsborough’s current listings. In early November, a 3,150 sq ft house on Bluebell Lane was asking $4.087 million and a 3,500 sq ft house on Eugenia Way was asking $5.28 million, while a 2,770 sq ft house on Lakeview Drive was pending near $4 million. On a half-acre lot, each would have room to grow on paper. Whether it can really grow depends on slope, setbacks and neighbors.
A height cap of 32 feet applies, within a sloping building envelope. Houses must be at least 2,500 sq ft, which matters on a teardown. And a home of 8,000 sq ft or more goes to the Town Council for final design review, unless that step is waived after a unanimous board vote with no objections.
Behind the house: courts and a second unit
A tennis court goes to the design review board, as do new houses and substantial additions.
A small second unit is often easier. Under the town’s ADU program, a unit up to 800 sq ft with 4-foot side and rear setbacks and a 16-foot height limit can go straight to a building permit. Junior units can reach 500 sq ft. For a family bringing a parent home, or an adult child back from college, that may do more than a major addition would, with far less process.
Toward the western edge and south of town, also check the fire map. State maps dated February 2025 show Moderate and High hazard zones there. New construction and teardowns in the town’s wildland-urban interface area must use fire-resistant methods and build defensible space into the landscape plan.
And inside?
Town documents describe design review in terms of exterior work, and we found nothing listing interior-only remodels. A kitchen or bath project may still need a building permit, so ask before you start. Inside is usually the freest part of a Hillsborough property, and often the best place to spend first.
Five calls before your contingencies end
- Ask the planning department what floor area that specific parcel allows.
- Ask for the permit history, and compare it with what you see on site.
- Have an arborist map any trees 18 inches or wider near your planned work.
- Look up the address on the fire hazard map, then get an insurance quote.
- Price your project and its timeline together, because a year of carrying costs on a home here is a serious number.
None of these takes long, and any one of them could change what you are willing to pay.
Mark Choey advises buyers and sellers in Hillsborough as a broker with Christie’s International Real Estate San Francisco. Listing details are as of November 9, 2026.
