Find out if and how you can build an Accessory Dwelling Unit on a Peninsula property — with city-specific size limits, setbacks, and approval rules.
California state law (AB 68, AB 881, etc.) sets a baseline that cities cannot fall below. But cities can be more permissive than the state minimum, and each Peninsula city has overlaid its own rules on top of state law. Atherton, for example, applies its 35-foot height limit and architectural review even to ADUs, which slows the process compared to Mountain View's near-ministerial approval. Always check the city, not just the state law.
An ADU (Accessory Dwelling Unit) is a fully separate dwelling with its own kitchen and bathroom, up to 1,200 sf detached or 50% of the primary home (whichever is larger). A JADU (Junior ADU) is a smaller unit (max 500 sf) carved out of the existing home with a separate entrance and a kitchenette. Most Peninsula cities allow one ADU + one JADU per single-family lot, plus optional garage conversions. State law allows this at minimum.
Yes, but only on the new value created. The original home's Prop 13 base remains protected. The ADU is added to the assessment at its construction cost or fair market value at completion. A $300K ADU build adds roughly $3,500/year to the property tax bill. The rest of your property is reassessed only on sale.
Mixed. ADUs add functional value (rental income or extended-family use) but typically do not return their full construction cost in resale. A $300K ADU build commonly returns $150K-$250K at sale. The exception is in walkable, high-rent neighborhoods (Old Mountain View, downtown Burlingame, Mountain View) where the rental income story justifies a higher buyer premium. In Atherton, Hillsborough, or Los Altos Hills, an ADU is more often a liability than an asset on a luxury estate.
Lisa works with several Peninsula architects who specialize in ADU design and can pre-vet a target property's buildable envelope before you write the offer.
Request an ADU Site Review