Yes, building an ADU in California does increase your property taxes, but only modestly, because the reassessment is limited to the ADU itself. Your existing home keeps its Proposition 13 base year value. For a typical ADU that costs around $200,000 to build, plan on roughly $2,000 or more in additional annual property taxes, depending on your county’s total tax rate.
What Actually Gets Reassessed
California’s property tax system runs on Proposition 13, the 1978 constitutional amendment that sets each property’s “base year value” at its purchase price or the value at the time of new construction. That base can rise by no more than 2% a year. The only events that reset it to current market value are a change of ownership or new construction.
Adding an ADU counts as new construction, not a change of ownership. That distinction is the whole ballgame. Your main home’s low Prop 13 base value is not touched. The assessor adds a separate value for the ADU on top of it, and the land’s assessed value stays put as well.
The base tax rate is 1% of assessed value, though most homeowners pay a bit more once voter-approved local taxes and special assessments are included.
How the County Assessor Values Your ADU
Under California Revenue and Taxation Code Section 70, new construction includes any addition to real property and any alteration that converts it to a different use. A detached backyard ADU qualifies. So does a garage conversion. Once the assessor’s office learns your ADU is complete, a staff appraiser sets a fair market value for the new improvement, and in practice that figure typically tracks what it cost you to build.
That value is then added to your existing Prop 13 base. If your home is currently assessed at $400,000 and the assessor values the ADU at $180,000, your new total assessed value is $580,000. The original $400,000 keeps its 2% annual cap. The $180,000 ADU value gets its own base year value, also capped at 2% growth per year going forward.
Garage Conversions Get a Small Break
Converting an existing garage rather than building from scratch works slightly in your favor. The assessor calculates the increment of market value the project added to the overall property. Because the garage already carried an assessed value as part of your home’s improvements, only the difference between the old garage value and the new ADU value is treated as new construction. A $120,000 garage conversion can add less to your assessment than a ground-up build of the same cost.
The Land Isn’t Reassessed
Your land’s assessed value doesn’t change when you add an ADU. Reassessment applies only to the new improvement. In California, where land often makes up a large share of a property’s assessed value, that matters. Keeping the land at its Prop 13 base saves real money over the long haul.
The Supplemental Tax Bill
After your ADU is completed, expect a one-time supplemental tax bill that covers the period between completion and the end of the current fiscal year on June 30. The county subtracts your old assessed value from the new assessed value, applies the tax rate to the difference, and prorates based on how many months remain in the fiscal year.
Say your ADU is finished in October and adds $200,000 in assessed value. The supplemental bill covers roughly nine months, October through June. At a 1% base rate, that’s about $1,500 for the initial period. Starting the following July 1, the ADU’s value shows up on your regular annual tax bill, and you pay taxes on the full new assessed value from there on. Before the supplemental bill lands, the county assessor mails a notice explaining the new valuation.
If You Think the Assessment Is Too High
You can challenge the assessor’s valuation, but the window is short. File an appeal with your county’s Assessment Appeals Board within 60 days of the date the supplemental assessment notice was mailed. If the 60th day falls on a weekend or holiday, the deadline moves to the next business day.
Miss that window and you aren’t completely locked out. You can still file during the regular annual assessment appeal period, which runs July 2 through September 15. There’s a real cost to relying on that backup, though. If you win a reduction through the regular appeal period after missing the supplemental deadline, the reduction only applies to the regular assessment roll going forward. You won’t get a refund on the supplemental taxes you already paid. Mark the 60-day clock on the day the notice arrives.
Impact Fees and Utility Charges
Property taxes aren’t the only ongoing cost tied to building an ADU, and California law limits several of the adjacent charges local agencies used to collect on new housing.
As of January 1, 2026, ADUs with 750 square feet or less of interior livable space are completely exempt from local development impact fees. Junior ADUs (JADUs) of 500 square feet or less are also exempt. If your ADU exceeds 750 square feet, impact fees must be charged proportionately, based on the ADU’s square footage relative to your primary home, rather than at the full rate a new house would face.
Utility connection fees are also restricted. Local agencies, special districts, and water corporations generally cannot treat an ADU as a new residential use when calculating water and sewer connection fees or capacity charges. The one exception is an ADU built at the same time as a brand-new primary home. School districts may charge impact fees on ADUs larger than 500 square feet, but ADUs of 500 square feet or less are exempt from school fees entirely.
Between the narrow scope of the reassessment and the fee exemptions on smaller units, the tax hit from adding an ADU in California is usually far smaller than owners expect going in. The number to plan around is the assessor’s value of the ADU itself, multiplied by your county’s total tax rate. Everything else on your property stays where Prop 13 left it.