Will your ADU trigger a property tax reassessment?

The single biggest tax myth about ADUs is that building one resets the assessed value of the entire property. It doesn't. Under Proposition 13 and the State Board of Equalization's Rule 463, only the new construction itself is added to the base year value. The rest of the property keeps its original Prop 13 assessment.

How the LA County Assessor calculates it

The LA County Assessor sends a field appraiser after the certificate of occupancy is issued. They establish a base year value for the new construction equal to its market value at completion. That value is added to your existing assessment. The tax rate applied includes the base countywide rate plus any voter-approved bonds specific to your tax rate area — look up the exact combined rate for your parcel on the LA County Assessor site.

Worked example: 800 sq ft detached ADU

• —Existing assessment — find your current Prop 13 base-year assessed value on your most recent county tax bill or the LA County Assessor's parcel lookup.
• —New construction value — the assessor sets this from the ADU's market value at completion, not its construction cost.
• —Combined assessed value — the new construction value is added to, not merged with, your existing Prop 13 base.
• —Resulting tax change — multiply the new construction value by your parcel's combined tax rate, available from the LA County Assessor, to estimate the added annual tax.

What's NOT reassessed

Routine maintenance, like-kind replacement, and the existing primary dwelling are all untouched. BOE Rule 463 lists the exclusions in detail.

Garage conversions: a special case

Converting an existing garage to an ADU is treated as new construction inside an existing structure. The reassessment is typically lower than a ground-up detached ADU because the assessor only values the conversion delta, not the original garage shell. The type comparison guide covers the structural trade-offs that drive that delta.