How this calculator works
Added yearly tax = value added for the new construction × overall rate ÷ 100.
What each input means
| Input | What it is actually asking |
|---|---|
| Value added | California assessors add the market value of the new construction, not necessarily its cost, according to county assessor guidance we cite. |
| Rate | The total ad valorem rate on your bill, including voter-approved debt. |
Worked examples
Including one where the naive answer misleads, which is the example most calculators leave out.
Reading your bill
Find the total rate on your current bill and type it. Fixed charges and special assessments are not included in the rate; add them separately.
Supplemental bill
In California, a one-time supplemental bill is often issued after completion, prorated to June 30, according to county assessors.
What changes the result
The added value drives the result. The assessor’s value may be above or below your cost.
Local considerations
Outside California, many states reassess the whole property. Ask your county assessor how new construction is valued.
When not to use this
Do not budget on this alone. See our ADU property tax research for the California rules in full.
Questions this calculator answers
- Does an ADU raise my property taxes?
- Usually, because new construction adds assessed value. In California only the new part is reappraised.
- Does California reassess my whole house?
- No, under Rev. & Tax. Code § 71 the rest keeps its base value.
- What is a supplemental assessment?
- A one-time bill for the part of the year after completion, issued by many California counties.
- Is a garage conversion new construction?
- A conversion to a different use can be, under Rev. & Tax. Code § 70. Ask your assessor.
Sources and methodology
Figures dated 27 October 2026. Last reviewed .
- Rev. & Tax. Code § 71 — new base year value for the new portion only (California Legislature, retrieved 2026-09-05)
- Rev. & Tax. Code § 70 — definition of newly constructed (California Legislature, retrieved 2026-09-05)
- How construction affects taxable value (San Mateo County Assessor, retrieved 2026-09-05)
- Notice of supplemental assessment (Orange County Assessor, retrieved 2026-09-05)
Related
- ADU property taxes (research) The California rules in full.
- ADU payback calculator Put the tax in yearly costs.
- Is an ADU worth it? The whole decision.