By ADU Scope editorial team · Last verified: October 2026
ADU property tax usually goes up in California, because the county assessor adds the unit’s market value to your assessment as new construction. However, only the new portion gets a new value. Your existing home keeps its current base-year value, and Proposition 13 still limits its annual growth.
This guide explains how that works for attached, detached and converted ADUs, and what a supplemental bill looks like. It also walks through a hypothetical example. It is general information, not tax or legal advice, and your county assessor makes the final call on your property.
How ADU property tax works under Proposition 13
California taxes property at 1 percent of its taxable value, according to the State Board of Equalization (BOE). Assessed value starts from a base-year value and rises with inflation, up to 2 percent a year. That changes when ownership changes or when new construction finishes. In that case, the new assessed value is the market value as of the date of the change or the date of completion, per BOE Publication 29.
Many tax bills also include voter-approved bonds and special assessments on top of the 1 percent. These charges vary by location, so check your last bill.
What counts as new construction
BOE says new construction includes any addition to real property. It also includes an alteration that amounts to a major rehabilitation or converts the property to a different use. For the new portion, the assessor sets a new base-year value. BOE adds that the base-year value of the remainder “shall not be changed.”
In short, the whole house does not reset to market value just because you add an ADU.
What BOE says about ADU property tax specifically
We looked for an ADU-specific BOE rule, handbook section or letter to assessors. We found none that sets out ADU treatment. BOE’s general new-construction page and Publication 29 do not mention ADUs by name.
However, BOE’s Legislative Division did address ADUs in its analyses of SB 1164 (2024). The analyses say that under existing law, ADU construction counts as new construction, and the assessor gives it its own base-year value. The rest of the property “retains its base year value.” That is the clearest BOE statement we found, but it is a bill analysis, not a rule. Please confirm your case with your county assessor.
County practice matches the general rule. For example, the Nevada County Assessor says it values new construction at market value on completion, and it leaves the existing land and structures at their current values.
Attached vs. detached ADU property tax
For tax purposes, the key question is what you built, not whether it touches the house. A detached ADU is a new structure. An attached ADU is an addition to the existing building. Either way, the general rule values the new space and leaves the old space alone.
The Nevada County page does not treat attached and detached units differently. Other counties might use different appraisal methods or documents, so ask your assessor which one applies to your plans.
Garage conversions and junior ADUs
A conversion adds less square footage, but it can still change your assessment. Under BOE’s general definition, an alteration that converts property to a different use can count as new construction. A garage turned into a home is a plausible example.
The Nevada County Assessor describes how it handles conversions with no added square footage. It estimates the increase in market value from the conversion, using several appraisal approaches, and adds that increase to the current assessed value. The whole property is not reassessed.
We did not find BOE guidance on junior ADUs built inside existing walls. Ask your assessor before you start work, because the answer shapes your budget.
Supplemental assessments: the bill that arrives after you finish
New construction does not wait for the next January 1 lien date. According to Publication 29, a supplemental assessment places the reappraisal into immediate effect. It is prorated, and an increase produces a supplemental tax bill.
In addition, the Nevada County Assessor says that when work is partly complete on January 1, the county can add a partial assessment to the annual bill. The final value follows completion as a supplemental event. As a result, you may see more than one bill change. Ask your assessor how and when your county sends them.
A hypothetical ADU property tax example
The numbers below are hypothetical. They are not an estimate for your home.
| Input (hypothetical) | Value |
|---|---|
| Existing home assessed value | $620,000 |
| Assessor’s market value of the new ADU | $180,000 |
| Total assessed value after completion | $800,000 |
| Assumed tax rate (1% base plus an assumed 0.15% local charges) | 1.15% |
| Annual tax on the existing home at that rate | $7,130 |
| Added annual tax from the ADU at that rate | $2,070 |
| Added tax per month | $172.50 |
At the 1% base rate alone, the ADU would add $1,800 a year, or $150 a month. The assumed local charges add $270 a year on top.
A supplemental bill is a one-time step, and its size depends on timing. Suppose the ADU finishes with nine months left in the fiscal year, and the county prorates by month. The supplemental tax would be about $1,552.50 ($2,070 × 9 ÷ 12). Counties may prorate differently, so treat that as a rough illustration.
Notice also that the assessor’s value is not your construction cost. It can be higher or lower, because it reflects market value. Compared with the $2,000 median new-ADU rent in a 2021 Terner Center survey of California owners, the extra tax in this example takes about 9 percent of that rent. That figure is dated, so use rents from your own market.
Is there an ADU property tax break?
There was a proposal. SB 1164 (2024) would have excluded ADU construction from the new-construction assessment. The February 2024 version ran until 15 years had passed since completion or a change in ownership. The bill did not become law. Its first hearing was canceled at the author’s request on June 24, 2024, and it went no further that session.
We did not find a later enacted ADU exclusion in the sources we checked, but we did not search every bill. Because the Legislature can revisit the idea, check current law with your assessor before you assume any exclusion applies.
How to estimate your own ADU property tax
Follow these steps:
- Ask your county assessor how it values new ADU construction, and whether your unit type (attached, detached or conversion) changes the method.
- Estimate the assessor’s market value for the new space. It may differ from your build cost.
- Multiply that value by your total tax rate. Your last tax bill shows the 1 percent base and local charges.
- Add the result to your ADU budget as a permanent annual cost.
- Ask when to expect a supplemental bill and how large it could be.
Next, put the tax into your project math. Our guide to how much an ADU costs in California and the ADU budget breakdown help you build the full cost picture. Then test whether rent can carry the debt using our ADU loan payment examples. Finally, see how lenders count rent in ADU rental income loan qualification.
Other tax effects to ask about
Rent is generally taxable income, and deductions and depreciation can change the result. Selling can trigger further tax questions. These topics depend on your facts, so talk to a CPA. For permit and zoning rules, see California ADU laws in 2026.
Frequently asked questions about ADU property tax
Does the whole house get reassessed when I build an ADU?
Under BOE’s new-construction rule, only the new portion gets a new value. The rest keeps its base-year value, so the answer is generally no.
Will ADU property tax rise right after completion?
Usually. A supplemental assessment can apply the increase from the completion date. Your county explains timing and billing.
Is there a 15-year exclusion for ADUs in California?
Not as far as we could verify. SB 1164 proposed one and died in 2024. Confirm current law with your assessor.
Do other states treat ADU property tax the same way?
No. This article covers California only. Check your own county assessor elsewhere.
General information, not tax or legal advice. Assessment practices vary by county and change over time. Confirm with your assessor and a tax professional.
Sources
- BOE, Publication 29: California Property Tax
- BOE, New construction
- BOE Legislative Division, SB 1164 bill analysis (May 16, 2024)
- BOE Legislative Division, SB 1164 bill analysis (Feb 14, 2024)
- Digital Democracy, SB 1164 status
- Nevada County Assessor, Accessory Dwelling Units
- Terner Center, statewide ADU owner survey (2021)