Does an ADU Increase Your Property Tax in California?

How does ADU property tax work in California?

When you add an ADU, the county assessor values only the new construction and adds that figure to your assessment. Your main home's Proposition 13 base year value is not touched. This is called blended assessment: the old house keeps its protected, lower assessed value, and only the ADU is taxed at current value. So an ADU that costs $250,000 to build typically adds roughly $2,500–$3,100 in annual property tax — not a reassessment of the whole property.

How do you estimate the added tax?

The added tax is roughly the ADU's assessed value times your local tax rate (about 1–1.25% in the Sacramento region, including voter-approved bonds). Assessed value usually tracks construction cost. Use the table to ballpark it, then confirm your exact rate on your county tax bill or with the Sacramento County Assessor.

See also:What an ADU costs in Sacramento — estimate the added value

When does the higher tax bill actually start — and what is a supplemental assessment?

The added tax doesn't wait for your next regular tax year. When the assessor records the ADU's new value, the county issues a supplemental assessment that prorates the increase from the date construction is completed through the end of the fiscal year — so you typically receive a one-time supplemental tax bill covering that partial year, then the full annual increase folds into your regular bill going forward. On the $250,000 ADU used above, the ongoing increase is roughly $2,500–$3,100 a year at a 1–1.25% rate; the first supplemental bill is just the slice of that amount for the months remaining in the year your unit is finished.

The rate that matters is your parcel's full effective rate, not a flat 1%. California's base rate is 1% of assessed value under Proposition 13, but most Sacramento-region parcels add voter-approved bonds and, in newer subdivisions, Mello-Roos community facilities district charges that push the effective rate toward the 1.25% top of the range used in the table. Those bond and Mello-Roos charges apply to the ADU's added value the same way the base rate does, which is why two identical ADUs can carry different annual tax depending on which district each home sits in. Check the line items on your existing tax bill to see your parcel's real rate before you estimate the ADU's added tax.

Why isn't your main house reassessed?

Proposition 13 caps annual increases in a property's assessed value and only allows full reassessment on a change of ownership or new construction (see the California State Board of Equalization). Adding an ADU is new construction, but the assessor applies it narrowly — only the ADU is the 'new' value. The existing home is unchanged in the eyes of the assessor, so its long-held, lower base year value carries forward untouched.

Is there a special ADU property-tax break?

No — California has no ADU-specific property-tax exclusion. A proposed one (SB 1164) stalled in the Legislature in 2024 and never became law, so don't budget around it. Your only protection is the standard Proposition 13 treatment described above: the assessor adds the ADU's new value and leaves your main home's base year value untouched.

In practice that standard treatment is already a meaningful break, even without a special carve-out. Because only the ADU's new value is assessed — not your whole property — the added tax is roughly 1–1.25% of the ADU's value rather than a reassessment of the entire home, which on a $250,000 unit means about $2,500–$3,100 a year instead of a far larger bill if your decades-old base year value were reset. The blended approach is the protection; a dedicated ADU exclusion would only trim an already-narrow figure. Confirm current treatment with your county assessor before you build.

See also:California ADU law in 2026 — statewide ADU changes

How does property tax affect your ADU's return?

The added property tax is a real holding cost, so it belongs in any honest ROI calculation. On a unit renting for $2,000/month, a $2,800 annual tax bill consumes a little over one month of rent — meaningful but small against gross income. It's one line in a stack of holding costs: alongside the tax, plan for landlord insurance and ongoing maintenance, which together typically run a few hundred dollars a month on a detached unit and pull your true net below the gross rent.

The encouraging part is the proportions. At roughly 1–1.25% of the ADU's value, the tax on a $250,000 unit lands near $2,500–$3,100 a year against rent of $1,500–$2,800 a month, so it trims your return without erasing it — which is how a $250,000 build still tends to pay back in about 8–12 years before appreciation and resale lift. The full payback math, including taxes, insurance, and management, lives in the ROI guide.

See also:ADU rental income & ROI in Sacramento

What are the most common property-tax mistakes?

• Assuming the whole home gets reassessed — only the ADU's value is added
• Forgetting to budget the added tax into monthly cash flow
• Confusing assessed value with market value — assessed value tracks construction cost
• Overlooking local bond rates that push the effective rate above 1%
• Treating this guide as tax advice instead of confirming with the county assessor