Buying Guide
ADUs and Property Taxes in California: What to Expect
Building an ADU in California does not reassess your entire property, only the new ADU is added to your assessed value. Thanks to Proposition 13, your existing home keeps its current (often much lower) assessed value, and the county adds a “blended” assessment for just the new construction. In practice, your annual property tax goes up by roughly 1 to 1.2% of the ADU’s construction cost, not 1 to 1.2% of your whole property’s new market value.
That distinction saves California homeowners a lot of money, and it’s one of the most misunderstood parts of building an ADU.
The short version: An ADU triggers a partial reassessment, only the ADU is taxed as new value, not your whole home. Budget roughly 1 to 1.2% of the ADU’s cost per year in added property tax. This is general information, not tax advice, confirm with your county assessor or CPA.
How the assessment actually works
California’s Proposition 13 caps how much your existing assessed value can rise each year (about 2%). When you add an ADU, the county assessor doesn’t throw that out. Instead they:
- Keep your home’s existing Prop 13 assessed value as-is.
- Add a new assessed value for just the ADU, based on its construction cost.
- Combine the two into a “blended” assessment.
So the only part taxed at current rates is the ADU itself.
A rough example
On a $287,000 ADU at a ~1.1% effective rate:
| Amount | |
|---|---|
| ADU construction value added | ~$287,000 |
| Estimated added annual property tax | |
| Your existing home’s assessment | Unchanged |
For most owners, the added tax is comfortably covered by rental income if the ADU is rented, and it’s a deductible expense against that rental income.
What doesn’t change
- Your existing home’s Prop 13 basis stays put, no full reassessment.
- Your existing tax bill for the main house isn’t re-rated.
- Permit fees are separate from property tax (and at Framework First, we build a permitting budget into your project price, with your exact permit costs confirmed for your property in your feasibility study).
Other tax angles worth a CPA conversation
- Rental income is taxable, but you can deduct operating costs, depreciation, and the mortgage interest tied to the ADU.
- Home-office use of an ADU may be partially deductible.
- Selling later: the added value can affect capital gains, though the primary-residence exclusion may still apply to your portion.
These depend on your situation, talk to a CPA. We’re builders, not tax advisors, so treat this guide as a starting map, not advice.
The bottom line
An ADU adds a modest, predictable amount to your property tax, based only on the ADU’s cost, not your whole home, and that cost is usually dwarfed by the rent and equity it generates. Use the ROI calculator (which already assumes ~1.25% property tax) to see the net picture for any model.
Frequently asked questions
Will building an ADU reassess my whole property?
No. Only the new construction is assessed and added to your existing bill. Your main home keeps its current Proposition 13 base.
Roughly how much more will I pay each year?
Plan on about 1 to 1.25% of the ADU’s cost per year. The ROI calculator already assumes roughly 1.25%, so the projections you run there are net of it.
Does the rent make up for the tax increase?
Almost always, many times over. The added tax is a predictable line item, while a well-placed ADU generates monthly rent and long-term equity against it.
Want the real numbers for your property and county? Start a feasibility study, we itemize the costs so there are no surprises.