Hillsborough is a town of estates, not subdivisions: large oak-studded lots, winding roads with no sidewalks, and no commercial district at all. The housing stock runs from historic mansions to mid-century ranches, all set on parcels measured in fractions of an acre at minimum and full acres at the top end. Hillside terrain, heritage oaks, and a fiercely protected residential character mean ADU work here is estate planning as much as construction.
On Hillsborough’s estate parcels, the natural path is a full-size detached ADU up to 1,200 square feet — a guest house, caretaker residence, or multigenerational cottage placed with the same care as the main home’s landscaping. These lots have the depth for a true second dwelling with private outdoor space, and the home values support a finished unit built to estate standards. Attached additions work where the main home’s envelope allows, and garage conversions suit properties where existing structures sit on favorable footprints.
Junior ADUs (JADUs) are a strong option in Hillsborough’s larger homes. Carving a compact, up-to-500-square-foot unit out of existing living space gives a homeowner a low-disruption path to housing for an aging parent, live-in help, or an adult child — no new foundation, no grading, and no exterior change to the property’s street presence. State law prohibits cities from requiring owner-occupancy for standard ADUs, so a standard detached or attached unit can be rented without the homeowner living on site, while JADUs do require a recorded deed restriction.
ADU projects in Hillsborough go through the Town’s Planning and Building Department, and California’s ministerial (by-right) approval process applies — a compliant project cannot be subjected to discretionary hearings. State law caps required side and rear setbacks at 4 feet. The genuinely local wrinkles are terrain and trees: hillside parcels often fall into fire-severity or wildland-interface review that adds defensible-space planning, and the town’s mature oak protections mean we confirm tree locations and root zones during feasibility before placing any detached structure.
Rental and multigenerational demand in Hillsborough is driven by estate-scale family living — grandparents close by, adult children, live-in help — with the Burlingame Caltrain station and the broader Peninsula employment corridor minutes away. Older estate homes often need electrical panel upgrades and sewer lateral evaluations before ADU construction, which we fold into the feasibility review. The unit should be designed to the standard of the estate it sits on: quiet, well-built, and permitted correctly the first time.
Ready to find out what your Hillsborough lot can carry? ADU Cities is a Southern California ADU design-build firm — we handle feasibility, design, permits, and construction under one roof. Call 562.810.1965 for a free consultation and site evaluation. CSLB #1138550.
