Published: 30 Oct 2026 · Last verified: 30 Oct 2026 · Next review: Jan 2027 This is general information about California property tax mechanics, not tax advice. Assessment practices vary by county — confirm with your county assessor.
The fear is simple: you build a $320,000 ADU in the backyard, and the county reassesses your entire property at 2026 market value, wiping out decades of Prop 13 protection. It does not work that way.
California’s property tax system, built by Proposition 13 in 1978, reassesses property only on specific trigger events — a change in ownership, or new construction. An ADU is new construction. But the reassessment applies to the new construction only, not the whole parcel. The assessor adds the value of the ADU to your existing base-year value. Your original house keeps its 2%-per-year cap, untouched.
The rule, in one paragraph
Property Tax Rule 463 (California Code of Regulations) is the governing regulation. When new construction is completed, the assessor establishes a new base-year value for only that portion of the property which is newly constructed. The existing improvement’s base-year value continues on its Prop 13 track. This is the same rule that covers room additions, second stories, and garage conversions — an ADU is not special in the assessor’s eyes.
The California State Board of Equalization (BOE) oversees Prop 13 implementation statewide; your county assessor applies it locally.
What actually gets added to your tax bill
The math has three parts:
- The ADU’s assessed value. The assessor determines this at completion — typically based on construction cost, but not always equal to what you paid. Market-based adjustments apply.
- The tax rate. About 1.1% of assessed value per year: the 1% Prop 13 base rate plus voter-approved local bonds and special assessments (roughly 0.1% statewide on average, per the Legislative Analyst’s Office; your tax rate area may differ).
- The 2% cap going forward. Once the ADU’s value is on the roll, it gets the same Prop 13 protection — annual increases capped at 2%.
A rough planning figure: an ADU assessed at $200,000 adds about $2,200/year to the property tax bill. An ADU assessed at $300,000 adds about $3,300/year. These are illustrations, not assessments — your county assessor sets the actual number.
What does NOT happen
- Your whole property is not reassessed. The original house keeps its existing base-year value. If you bought in 1995, that 1995 basis (plus 2%/year compounding) survives the ADU build.
- There is no countywide reappraisal. California does not do periodic mass revaluations. The assessor only acts on trigger events.
- Selling later resets everything. If you sell the property, the buyer gets a new base-year value at the purchase price — ADU included. Your Prop 13 protection does not transfer (with limited Prop 19 exceptions for parent-child transfers of a primary residence).
The timing question
New construction is assessed as of the date of completion. If your ADU finishes in 2026, the added value hits the roll for the following assessment cycle. A supplemental assessment may apply for the remainder of the current fiscal year — budget roughly 1.1–1.3% of the ADU’s assessed value as a planning figure for the supplemental bill, then adjust when the real notice arrives.
What to do before you build
- Call your county assessor’s office. Ask how they value ADU new construction and what documentation they want. Practices vary by county.
- Budget the ongoing cost. Property tax on the ADU is a permanent annual line item — add it to your rental income math alongside insurance and maintenance.
- Keep construction records. The assessor may ask for permits, contracts, and final costs. Clean paperwork makes the valuation conversation shorter.
- Don’t confuse this with the 2% cap. Your existing house’s assessed value keeps growing at most 2%/year regardless of what the ADU costs to build.
The bottom line
An ADU raises your property tax by roughly 1.1% of the ADU’s assessed value per year — a real, permanent cost, but a fraction of what a full reassessment would be. Prop 13’s new-construction rule is one of the most owner-friendly parts of California tax law: the ADU gets taxed, the house you already owned does not get re-taxed. Price it into the pro forma, then build.
When exactly does the tax clock start? The completion-date rule
New construction is not taxed from the permit date, the first inspection, or the day you sign the contract. It is taxed from the date of completion — and California defines that date precisely for supplemental assessments.
Property Tax Rule 463.500 (California Code of Regulations, title 18) sets the date of completion of new construction for supplemental assessments as the earliest of:
- the date the new construction is available for use by the owner, or
- if the owner does not intend to occupy or use the property (or a specified portion of it) and notifies the assessor in writing before, or within 30 days after, the date construction commenced, the date the property is actually occupied or used by the owner or with the owner’s consent.
If the assessor never receives that written notice, the completion date is conclusively presumed to be the date the construction is available for use. Rule 463 adds the practical test for “available for use”: the date of the final inspection by the appropriate government official, or, if there was no such inspection, the date the prime contractor fulfilled all contract obligations.
In plain terms: the tax clock usually starts at final inspection, not at move-in. If you are building an ADU to rent out and will not occupy it yourself, the written-notice rule is worth knowing before construction starts — the 30-day window runs from commencement, not from completion. (Permit timing itself is a separate clock: see our ADU permit timeline for the 60-day ministerial rule.)
The supplemental bill: what actually arrives
Completion also triggers the supplemental assessment system (Revenue and Taxation Code sections 75.10–75.12). On completion of actual physical new construction, section 75.10 directs the assessor to appraise the new construction at its full cash value on the completion date; that value becomes the new base-year value for the new portion only.
Section 75.11 then decides how many supplemental assessments hit the supplemental roll:
| Completion date | Supplemental assessments |
|---|---|
| 1 January – 31 May | Two: one against the current roll, and a second for the value change due to the new construction against the roll being prepared |
| 1 June – 31 December | One: the difference between the new base-year value and the taxable value on the current roll |
Each supplemental assessment is prorated for the part of the fiscal year remaining after completion, so a mid-year completion produces a partial-year bill first, and the full annual amount lands on the regular roll afterwards. Expect the supplemental notice to arrive separately from your regular tax bill, and budget for it in the completion year — it is the most commonly missed line in an ADU pro forma.
New detached ADU vs. garage conversion: the tax comparison
Both are new construction. The difference is what value the assessor adds, and whether anything is subtracted first.
| New detached ADU | Garage conversion ADU | |
|---|---|---|
| Trigger | New construction (addition) | New construction (alteration / change in use) |
| What is added | Full cash value of the new unit at completion | Value of the converted space at completion |
| What is subtracted | Nothing — the existing house keeps its base-year value | The factored base-year value of the garage portion removed or replaced during construction, if a substantial portion was removed (Rule 463: the new value is added to the pre-existing taxable value reduced for removed property) |
| Completion-date rule | Rule 463.500: available for use (final inspection in practice) | Same Rule 463.500 test |
| Supplemental assessment | Sections 75.10–75.11, as above | Same sections, same timing |
| Ongoing treatment | New portion capped at 2%/year like everything else | Same 2% cap on the new portion |
The practical consequence: a garage conversion’s net tax addition can be smaller than a same-size detached unit’s, because part of the converted space was already on the roll as a garage. But the conversion is still a reassessment event for the new portion — “we reused the shell” does not mean “no new tax.”
Worked example: a $100,000 ADU assessment
Suppose the assessor sets the new construction’s base-year value at $100,000. At California’s 1% Prop 13 base rate, that is:
$100,000 × 1% = about $1,000 per year in base tax.
That is the base rate only. Voter-approved local bonds, parcel taxes, and special assessments sit on top and vary by tax rate area — the planning figure of about 1.1% used earlier in this post (roughly $1,100/year on the same $100,000) includes a statewide-average allowance for those. If the ADU is completed mid-fiscal-year, the first supplemental bill is prorated to the months remaining in that fiscal year rather than a full $1,000.
Your county assessor sets the actual value, and your tax rate area’s voter-approved rates set the actual add-ons. Treat $1,000 as the arithmetic floor for a $100,000 assessment, not as a quote. How we verify statutes and rules before publishing is documented in our Methodology.
Update log
- 31 Oct 2026: Added the completion-date rule (Property Tax Rule 463.500), the supplemental assessment mechanics (Rev. & Tax. Code §§75.10–75.11), the new-ADU vs. garage-conversion comparison table, the $100,000 worked example, and one FAQ on when the tax increase starts. No earlier sentences were changed.