The short answer: partial reassessment, not a full one
Under Proposition 13, your home's existing assessed value is protected and doesn't get reassessed just because you build an ADU. Instead, the county assessor adds the ADU as new construction, assessed separately at its own market value as of the date it's completed. Your original home keeps its existing Prop 13 base-year value; the ADU gets its own new value added on top.
How the assessor actually calculates it
Illustrative example only — not a quote
Your annual property tax bill is based on this new total assessed value — generally around 1% of assessed value, plus any local voter-approved bonds and assessments, and subject to a maximum 2% annual increase on the existing portion going forward. The $120,000 figure above is illustrative; your assessor typically bases the ADU's assessed value on its actual construction cost or comparable market value, not on the general cost estimates in our own ADU cost guide.
Estimate your own increase
ADU property-tax impact estimator
Your assumption — assessors typically base this on your actual construction cost or comparable market value, so your build budget is a reasonable starting point.
The estimate above uses only the 1% statewide base rate. Your actual bill also includes any local voter-approved bonds and special assessments, which vary by county and district — your county assessor and most recent property-tax bill are the authoritative sources for your total effective rate.
What triggers the reassessment
- Building permit completion. Once your ADU passes final inspection, the county typically becomes aware of the new construction through the permit record.
- Declaration of new construction. Assessors generally mail a form requesting your actual construction costs, which helps them establish the ADU's assessed value.
- Only the new square footage. A garage or interior conversion into an ADU may be assessed differently than new ground-up construction, since you're not adding new square footage to the property in the same way — confirm with your local assessor how conversions are treated.
This is general information, not tax advice
Property tax assessment rules and how they're applied can vary by county and by project type. This page describes the general statewide framework, not a specific number for your property. Your county assessor's office is the authoritative source for how your specific ADU will be assessed.
Does this affect my mortgage or insurance too?
A higher assessed value generally means a higher annual tax bill, which can affect your monthly mortgage payment if your taxes are escrowed. It's a separate question from whether the ADU increases your home's resale value — that's a market question your appraiser or a real estate agent can speak to, while the tax reassessment is a fixed, calculable outcome of the construction itself.
Frequently asked questions
No. Under Proposition 13, only the new construction — your ADU — gets a new assessed value added to your property. Your existing home retains its current Prop 13 base-year assessed value.
Assessors generally use the ADU's market value as of its completion date, often informed by the actual construction costs you report on a declaration of new construction form the county mails you.
It can be assessed differently since you're converting existing space rather than adding new square footage in the same way. Confirm the specifics with your county assessor's office, since practices can vary.