Building an ADU does not reassess your whole home — the county only taxes the value of what you built.
It is the fear that stops a lot of ADU projects before they start: “If I build this, will the county reassess my whole house and blow up my property taxes?” The answer is no — and once you see how California actually taxes an ADU, the increase is a lot less scary than the rumor.
Your Whole Home Does Not Get Reassessed
Your whole home does not get reassessed when you add an ADU. Proposition 13 still protects your existing base value — the assessed value your main home has carried, often far below today’s market value if you have owned it a while, stays exactly where it was. Building an ADU is not a change of ownership, and it does not reset your home to current market value.
What the County Actually Taxes: Only the New Construction
The county only adds the value of the new construction to your assessment. You are taxed on what you built — not on a brand-new value for your entire property. The assessor values the ADU once it is built, typically looking at its cost and size, and that added value is combined with your existing assessment. Assessors call the result a blended assessment: your original home keeps its old Proposition 13 base, and the ADU’s value is layered on top with its own base year.
How Big Is the Increase, Really?
For most homeowners it is a modest increase based on the cost and size of the ADU, rather than a shocking jump all at once. The rough math: California’s base property tax rate is about 1% of assessed value, plus local voter-approved assessments. So if the assessor adds $200,000 of value for a new ADU, the increase is on the order of $2,000 a year at that base rate — real money, but a fraction of what a full reassessment of the whole property would cost, and usually a fraction of the rent the unit brings in. Our ADU rental income guide runs that side of the math.
Garage Conversions Usually Add Less
A garage conversion usually adds less value than a large detached ADU, because less new construction is involved — the walls, roof, and foundation already exist and were already part of your assessment. The assessor is mainly valuing the improvement you made to an existing structure, and that keeps the added tax lower. It is one more reason garage conversions are the budget-friendly end of ADU building, on both the construction bill and the tax bill.
No Surprises: Estimate It Before You Build
At ADU Cities, we help you estimate that increase before you build, as part of the feasibility conversation — what you are likely to add in assessed value, based on the type and size of unit your lot can carry. Your county assessor makes the final call on your specific parcel, so for exact figures on your property, they are the authority. But no homeowner should walk into a final inspection guessing what happens to their tax bill afterward.
We will look at your property, the type and size of ADU it can carry, and what that is likely to add to your assessment — free consultation, no obligation.
Call 562-810-1965 Get a Free ConsultationGeneral information about how California assesses ADUs under Proposition 13 and Revenue and Taxation Code Section 70 (new construction), not tax advice. Assessors value each parcel individually; confirm figures for your property with your county assessor or tax professional.
