Does an ADU Increase Property Taxes?

Yes, building an ADU will increase your property taxes, but only on the value the ADU adds, not on your entire property. California’s Proposition 13 rules mean your existing home keeps its current assessed value, and the county simply adds the new construction value of the ADU on top of it.

The short version: An ADU triggers a partial reassessment, not a full one. The county adds the ADU’s new construction value to your existing assessed value, and your original home’s assessment stays exactly where it was.

How the reassessment actually works

Most California homeowners are used to a scary idea: build something new, and the county reassesses your whole property at today’s market value. That is not what happens with an ADU.

Under Prop 13, your primary home keeps the assessed value it already has, which is often far below current market value if you have owned the home for years. When you add an ADU, the county assessor only calculates the added value of the new structure and tacks that onto your existing assessment. Your original home’s number does not move.

In practical terms, that means the property tax increase is tied to what the ADU itself is worth as new construction, not to what your whole property might sell for today.

What determines the size of the increase

The added assessed value generally reflects the cost and scope of the ADU itself, things like square footage, finish level, and site work. Every county assessor’s office calculates this a little differently, and the exact dollar increase depends on your specific parcel, your county’s assessment practices, and current tax rates in your area.

Because these numbers are specific to your parcel and change over time, we are not going to guess at a dollar figure here. Your county assessor’s office is the correct source for exact numbers, and a Framework First feasibility study will also help you understand cost context before you commit.

Where to get the full breakdown

We cover the mechanics of ADU property tax reassessment in a lot more depth, including how the process typically unfolds after your ADU is complete, in our dedicated guide: How ADUs Affect Property Taxes in California. If property tax is a major factor in your decision, that is the better next read.

What this means for building with Framework First

A partial reassessment is a manageable, predictable cost of adding real living space and real value to your property. It should not be a reason to avoid building. Most homeowners find the added rental income, resale value, or space for family far outweighs a modest increase in the tax bill.

If you want a clear picture of what an ADU would cost and add to your property before you commit to anything, our feasibility study is the right starting point. It looks at your specific lot and gives you real numbers to work with, not guesses. You can also browse our ADU models to see what fits your property and budget.

Frequently asked questions

Will my whole property be reassessed at market value once I build an ADU?

No. Only the new construction value of the ADU gets added to your assessment. Your existing home keeps the assessed value it already had under Prop 13, so your original tax base is protected.

When does the reassessment happen?

The county assessor typically reassesses once the ADU is complete and a certificate of occupancy or final inspection has been recorded. Timing details can vary by county, so your local assessor’s office is the best source for your specific process.

Does a JADU get reassessed the same way as a detached ADU?

The same general principle applies: only the new construction value gets added, not the whole property. JADUs are usually created within the existing walls of the home, so the added value is often smaller than a full detached ADU. Confirm specifics with your county assessor.

Can I find out roughly what my property taxes will look like before I build?

Your county assessor’s office can give you the clearest sense of how they handle ADU assessments locally. A Framework First feasibility study also helps you understand the full cost picture for your specific lot before you commit to building.

Does financing my ADU change how it’s taxed?

No. Property tax reassessment is based on the new construction value of the ADU itself, not how you paid for it. Whether you pay cash or finance, the assessment mechanics work the same way.

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Ready to see what an ADU would actually cost and add to your property? Start your feasibility study and get real numbers for your lot.