California ADU Property Tax 2026: What Prop 13 Protects, What Gets Reassessed, and What to Ask Before You Build

Building an ADU does not trigger a full reassessment of your home under Proposition 13. Your existing home's assessed value is protected. What changes is your tax bill increases by the assessed value of the new ADU — which is determined by the county assessor based on the ADU's value as new construction. Understanding this distinction matters before you start budgeting your post-construction carrying costs.

How ADU Property Tax Actually Works Under Prop 13

Proposition 13 (1978) established two key rules that govern California property taxation: assessed values are based on purchase price, and annual increases are capped at 2% per year. A property can only be fully reassessed at current market value when it changes ownership or undergoes "new construction."

Adding an ADU constitutes new construction — but only the ADU portion triggers reassessment. California Revenue and Taxation Code §70 provides that only newly constructed property is subject to reassessment. The existing primary residence retains its Prop 13-protected assessed value, which may be substantially below current market value for homeowners who purchased many years ago.

What "Assessed at Current Market Value" Means in Practice

County assessors use one of two methods to value new ADU construction:

Cost Approach (Most Common for New Construction)

The assessor calculates the replacement cost of the structure — materials and labor to build an equivalent ADU — then adjusts for any depreciation (minimal for new construction). This often produces values close to the actual construction cost, though assessors use their own cost schedules rather than your specific contractor bids.

Comparable Sales Approach

Some assessors look at comparable ADU sales in the area. Where ADU sales data is limited — which is still the case in many California markets — assessors typically fall back to the cost approach.

An illustrative example of how the math works:

The base property tax rate in California is 1% under Prop 13, but voter-approved bonds, special assessments, and local measures typically bring the effective rate to 1.1%–1.3% depending on the jurisdiction. Use your current tax bill's effective rate, not just 1%, for your projections.

SB 1164 — What It Was, and Why It Matters That It Died

SB 1164 would have excluded the assessed value of a new ADU from property tax for 10 years from the date of completion. The bill passed the California State Senate in 2024 and advanced through Assembly committees before failing to make it to a floor vote before the legislative deadline.

This is worth flagging explicitly because the bill received significant press coverage in 2023–2024, and many homeowners still believe the exemption passed. It did not. If it is reintroduced and passes in a future session, it would be a benefit — but count on that only when it's signed law.

The Supplemental Tax Bill — What to Expect

When your ADU receives its certificate of occupancy (or when the permit is finalized, depending on your county's process), the county assessor will add the ADU's assessed value to your property's assessment roll. This triggers a supplemental assessment — a one-time catch-up bill for the period from permit finalization to the next July 1 fiscal year start.

Key facts about supplemental bills:

• They typically arrive 6–12 months after the certificate of occupancy — not immediately
• They cover only the partial year from when the ADU was assessed to the end of the fiscal year (June 30)
• A second supplemental bill may also be issued if the assessment straddles two fiscal years
• The amount is prorated for the partial year — not a full year's tax on the ADU value
• After the supplemental period, the ADU value is folded into your regular annual tax bill starting the following fiscal year

Budget for a supplemental bill arriving roughly 6–12 months after your ADU is complete. The amount will be your ADU's assessed value multiplied by your effective tax rate, prorated for the months remaining in that fiscal year. If your ADU is assessed at $190,000 and your effective tax rate is 1.2%, the full annual add is $2,280 — a supplemental bill for a 7-month partial year would be approximately $1,330.

Mello-Roos and Special Assessments

Mello-Roos (Community Facilities Districts) and other special assessments are levied at the parcel level, not based on assessed value. These assessments fund infrastructure, schools, or community facilities and are typically fixed dollar amounts per parcel or per square foot of structure.

When you add an ADU:

• Mello-Roos district charges that apply per parcel are unaffected — you still pay the same parcel-level charge
• Assessments calculated by square footage of residential structure may increase proportionally with the ADU's square footage — check your district's rate formula
• School impact fees (which ADUs over 750 sq ft may be subject to) are a one-time development fee, not an ongoing assessment

Review your current property tax bill's line items to identify which assessments are per-parcel (unchanged) versus square-footage-based (potentially increased by the ADU).

What to Ask Your County Assessor Before You Build

You can call your county assessor's office before construction and ask directly how they'll value an ADU on your property. Most assessors will give you a general answer without committing to a specific number. Useful questions:

• "How does your office value new ADU construction — cost approach or comparable sales?"
• "What cost schedule do you use per square foot for residential construction in this area?"
• "Is there any local exemption, deferral program, or reduced assessment for ADUs?"
• "When does the supplemental assessment typically issue — at permit finalization or certificate of occupancy?"
• "Are there any pending changes to how ADUs are assessed in this county?"

Some county assessors' websites publish their construction cost schedules — ask for the relevant schedule for your property type.

County Assessor Contacts for Major California Counties

• Los Angeles County Assessor assessor.lacounty.gov — parcel search, assessment lookup, and new construction assessment information
• San Diego County Assessor/Recorder/County Clerk sdarcc.gov — property assessment and supplemental tax information
• Santa Clara County Assessor assessor.sccgov.org — includes new construction assessment guidance for ADUs
• Alameda County Assessor acassessor.org — East Bay properties including Oakland, Berkeley, Fremont, and surrounding cities
• Orange County Assessor ocassessor.gov — parcel assessment data and new construction information for all OC cities

Frequently Asked Questions