Yes, an ADU does increase property taxes in California, but only by the value of the ADU itself. Your main home’s assessed value stays locked in at its existing Proposition 13 base. For a typical ADU costing around $200,000 to build, plan on roughly $2,000 or more in added property tax each year, depending on your county’s total tax rate.
Why Your Existing Home’s Assessment Doesn’t Change
California property taxes run on Proposition 13, the 1978 constitutional amendment that fixes 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% per year, regardless of what the market does. Only two events reset a property to current market value: a change of ownership, or new construction.
Building an ADU is new construction, not a change of ownership. The protection that matters here is that your original home’s low Prop 13 base value is left alone. The assessor touches only the ADU.
The base tax rate is capped at 1% of assessed value, though most homeowners pay a bit more once voter-approved local taxes and special assessments are added in.
How the County Values Your ADU
Under California Revenue and Taxation Code Section 70, new construction covers any addition to real property and any alteration that converts property to a different use. A detached backyard ADU and a garage conversion both qualify. Once your ADU is complete, an appraiser from the assessor’s office determines the fair market value of the new improvement, and in practice assessors typically rely on what the project cost you to build.
That value is then added to your property’s 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 continues under its existing 2% annual cap, and the $180,000 gets its own base year value with its own 2% cap going forward.
Garage Conversions
Converting an existing garage works slightly in your favor. The assessor measures the “increment of market value” the project added, not the full value of the finished unit. 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 counts as new construction. A $120,000 garage conversion often adds less to your assessment than a $120,000 ground-up build.
The Land Isn’t Reassessed
Your land’s assessed value does not change when you add an ADU. Reassessment applies only to the newly constructed improvement. In California, where land is often a large share of a property’s total assessed value, keeping the land at its Prop 13 base matters.
The Supplemental Tax Bill
After the ADU is finished, you’ll receive a supplemental tax bill covering the gap between the completion date and the end of the current fiscal year on June 30. It’s a one-time, prorated charge. The county subtracts your property’s 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 completed in October and adds $200,000 in assessed value. The supplemental bill covers about nine months, October through June. At the 1% base rate, that works out to roughly $1,500 for that 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 then on. The assessor mails a notice explaining the new valuation before the supplemental bill lands.
Appealing the Assessed Value of Your ADU
If you think the assessor overvalued the ADU, the window to push back is narrow. 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, you get the next business day.
Miss that window and you aren’t entirely out of options. You can still file during the regular annual appeal period, July 2 through September 15. The catch is costly: if you win a reduction through the regular period after missing the supplemental deadline, the lower value applies only 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 your calendar the day the notice arrives.
Fees That Aren’t Property Taxes but Still Hit the Project
Property tax is only one line in the ADU budget. Local governments and special districts often charge development impact fees on new housing, and California law carves out meaningful relief for smaller units.
As of January 1, 2026, ADUs with 750 square feet or less of interior livable space are completely exempt from local impact fees. Junior ADUs of 500 square feet or less are also exempt. If your ADU is larger than 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 pay.
Utility connection fees are limited too. 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 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 anything 500 square feet or less is exempt from school fees entirely.