By Dylan Bertet, owner of Build Better Properties (CSLB #1077286), a Bay Area design-build contractor.
Short answer: yes, but only for the part you build. Under California’s Proposition 13, an addition doesn’t reset the assessed value of your whole house. The county assessor adds the value of the new construction on top of your existing assessed value, and the rest of your home keeps its original base year value.
It’s the question I hear most from homeowners thinking about adding on, usually right after a neighbor told them their tax bill would triple. It won’t.
How the reassessment works
When you finish an addition, the assessor values the new construction, for example the added square footage, at what it adds to your home’s market value. That amount gets its own base year and is added to your existing assessed value. Your original Prop 13 protection on the existing house stays in place.
The value the assessor adds isn’t always the same as what you paid to build. It’s based on what the new construction adds to the home’s market value.
What usually gets reassessed
- Added square footage, like a new bedroom suite, a bigger kitchen or a second story
- Converting space into something new, like turning a garage or attic into a bedroom
What usually doesn’t
- Normal maintenance and repairs
- Like-for-like replacements, such as a new roof, new cabinets or new fixtures
What’s excluded by law
- Qualifying solar energy systems
- Qualifying seismic retrofits
- Accessibility improvements for a disabled occupant (these require filing with the assessor)
Why this matters when you compare adding on with moving
Buy a bigger house and the whole purchase price becomes your new assessed value. Build an addition and only the new construction is added. For many Bay Area families that difference alone is a big part of why adding on beats moving.
I’m a builder, not a tax advisor. Your county assessor’s office can answer questions about your specific project.
Planning an addition? Read our full Home Additions FAQ, see the cities we serve, or call (650) 801-2699 to have us walk your house.