How Much Does an ADU Raise Your Property Taxes in California?
An accessory dwelling unit is new construction under the Revenue and Taxation Code, so the county assessor adds the ADU's value to your assessment when it is finished. It is not a reassessment of your home: the house and land keep the Proposition 13 value they had, and only the new unit gets a current base-year value. This page gives the arithmetic by county, the statute behind each step, and the cases that trip people up: conversions, unpermitted units and solar. The state rules on what you may build are on the California ADU laws page.
Quick answer
Added annual tax ≈ the ADU's assessed value × your area's tax rate. At the statewide-typical 1.1% to 1.25%, a $150,000 ADU adds about $1,650 to $1,900 a year, a $250,000 ADU about $2,750 to $3,100. The main house is not reassessed (§71). The increase starts on completion with a prorated supplemental bill (§75.10 and following), then grows by at most 2% a year.
What an ADU adds in 12 counties
| County | BOE average rate (2024-25) | $150,000 ADU | $250,000 ADU | $400,000 ADU |
|---|---|---|---|---|
| Los Angeles | 1.181% | $1,772 / yr | $2,953 / yr | $4,724 / yr |
| San Diego | 1.171% | $1,757 / yr | $2,928 / yr | $4,684 / yr |
| Orange | 1.064% | $1,596 / yr | $2,660 / yr | $4,256 / yr |
| Riverside | 1.173% | $1,760 / yr | $2,933 / yr | $4,692 / yr |
| San Bernardino | 1.127% | $1,691 / yr | $2,818 / yr | $4,508 / yr |
| Santa Clara | 1.193% | $1,790 / yr | $2,983 / yr | $4,772 / yr |
| Alameda | 1.220% | $1,830 / yr | $3,050 / yr | $4,880 / yr |
| Sacramento | 1.155% | $1,733 / yr | $2,888 / yr | $4,620 / yr |
| Contra Costa | 1.151% | $1,727 / yr | $2,878 / yr | $4,604 / yr |
| Fresno | 1.213% | $1,820 / yr | $3,033 / yr | $4,852 / yr |
| San Francisco | 1.174% | $1,761 / yr | $2,935 / yr | $4,696 / yr |
| Ventura | 1.116% | $1,674 / yr | $2,790 / yr | $4,464 / yr |
Dollar figures are the printed rate times the ADU value, rounded to the dollar; the rate is shown to three decimals so the arithmetic reproduces. Rates are each county's average effective ad valorem rate as published by the Board of Equalization (total levies ÷ net assessed value) for 2024-25; your Tax Rate Area's rate can be higher or lower, and Mello-Roos and parcel taxes are usually flat per parcel and do not change with the ADU. Every county's rate is on the rate-by-county page; for a ZIP-level rate use the by-ZIP tables.
The rule: only the new construction is assessed (§70, §71)
- What counts as new construction. §70 defines “newly constructed” as any addition to real property since the last lien date, and any alteration that converts the property to a different use or is the substantial equivalent of new. A detached ADU is an addition; a garage or basement conversion is a change of use. Normal maintenance, repair and like-for-like replacement are not new construction (Property Tax Rule 463(b)(4)).
- Only that portion gets a new value. §71 directs the assessor to determine the new base-year value for the portion of the property that was newly constructed, and to leave the rest at its factored base-year value. Building an ADU is therefore not a reason for the assessor to revalue your house.
- Date and value. New construction is valued at its market value as of the date of completion (§110.1); work in progress on a January 1 lien date is assessed at its value on that date (§71). Property Tax Rule 463 and Assessors' Handbook Section 410 govern the assessors' method, which typically starts from permitted cost.
- The bill. A supplemental assessment under §75.10 and following charges the added value for the months left in the fiscal year; from the next roll the ADU's value is part of the regular bill and grows by at most 2% a year. The supplemental calculator estimates the one-time bill.
- Your right to appeal the new value runs 60 days from the notice (§1605(b)), or from the tax bill in Los Angeles and other counties that have adopted §1605(c); see the appeal guide.
Conversions, unpermitted units and solar
| Case | Assessed? | Statute |
|---|---|---|
| Detached new-build ADU | Yes — its full value added on completion | §70(a), §71 |
| Garage, basement or attic converted to a JADU or ADU | Yes — the value the conversion adds, not the whole structure | §70(a) (change of use) |
| Replacing the roof, windows or systems of an existing unit like for like | No — maintenance and repair | Rule 463(b)(4) |
| ADU built without permits, found later | Yes — an escape assessment for up to four prior years at each year's rate | §531, §532(a) |
| Active solar system on the ADU, completed before January 1, 2027 | No — excluded until the property changes hands | §73 |
| Selling the property after the ADU is built | Everything is reassessed at the sale price, ADU included | §60, §110.1 |
The escape assessment guide covers the look-back and the four-year payment plan, and the solar exclusion page quotes §73 and its 2027 sunset.
Frequently Asked Questions
Does building an ADU reassess my whole house in California?
No. Under Revenue and Taxation Code §71 the assessor determines a new base-year value only for the portion of the property that was newly constructed. The existing house and land keep their factored Prop 13 value. Your bill goes up by roughly the ADU's assessed value times your Tax Rate Area's rate, which is about 1.1% to 1.25% in most of California before Mello-Roos and parcel taxes.
How does the assessor value a new ADU?
At the market value the construction adds as of its completion date (§71, §110.1). In practice assessors start from permitted construction cost and adjust toward what a buyer would pay for the added unit; a detached new-build ADU is usually valued near its cost, while a garage conversion is valued at the value the conversion adds, not the cost of the whole garage. The assessor mails a notice of the new value, and you may appeal it within 60 days of that notice (§1605(b)), or of the tax bill in Los Angeles and other §1605(c) counties.
When does the higher tax start?
On the date the ADU is completed. A supplemental assessment (§75.10 and following) bills the added value for the months left in the fiscal year, and from the next July 1 the ADU's value is part of your regular bill, growing by at most 2% a year like the rest of the assessment. The supplemental property tax calculator estimates the one-time bill.
Does a garage conversion or a JADU count as new construction?
Yes. §70 defines new construction to include any alteration that converts property to a different use or amounts to a substantial equivalent of new, and converting a garage or part of the house into a dwelling unit is a change of use. Only the value added by the conversion is assessed; normal maintenance and like-for-like repairs are not new construction (Property Tax Rule 463(b)(4)).
What if the ADU was built without a permit?
The assessor can still assess it when discovered, as an escape assessment for the years it was missed: four years back as the rule (§532(a)). Counties learn of unpermitted units from aerial imagery, sale listings and neighbor reports, and the escape bill carries each prior year's rate plus any interest. Permitting also matters for the state ADU rules; the ADU laws page covers them.
Do solar panels on the ADU add to the assessment?
Not if the system is completed before January 1, 2027. §73 excludes active solar energy systems from "newly constructed" for lien dates through 2025-26, and systems that qualify before 2027 stay excluded until the property changes hands. The solar exclusion page quotes the statute.
Learn More
Sources: Revenue and Taxation Code §§60, 70, 71, 73, 75–75.80, 110.1, 531–532 and 1605, Legislature's bulk data (build of October 18, 2026), paraphrased with the section cited; Board of Equalization Property Tax Rule 463 and Assessors' Handbook Section 410 (new construction). County rates: BOE county-average effective rates, 2024-25, as held in this site's data (see the rate-by-county page for sources). Figures are illustrations of the statute's arithmetic, not an assessment; the assessor sets the ADU's value. Educational information, not legal or tax advice.
Last reviewed: October 2026.