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Vol. I ·California ADU Desk ·Licensed · Bonded · Insured ·CSLB #1098432 ·BBB A+ Accredited ·120+ ADUs Delivered ·Starting $250K ·10–16% ROI ·Los Angeles ·Bay Area ·Vol. I ·California ADU Desk ·Licensed · Bonded · Insured ·CSLB #1098432 ·BBB A+ Accredited ·120+ ADUs Delivered ·Starting $250K ·10–16% ROI ·Los Angeles ·Bay Area ·
GoldenStateADU

Finance · 7 min read · April 2, 2026 · 303 words

Will your ADU trigger a property tax reassessment?

Under Prop 13 and BOE Rule 463, only the new construction value is reassessed — not the entire property. A practical walkthrough of how the LA County Assessor calculates the bump.

Key takeaways

  • Existing property assessed at $620,000 (Prop 13 base) — taxed at ~$7,200/yr.
  • New 800 sq ft ADU completed; assessor values the addition at $280,000.
  • New total assessed value: $900,000.

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 annual tax on the addition is roughly 1.16% of that value (countywide rate plus voter-approved bonds).

Worked example: 800 sq ft detached ADU

  • Existing property assessed at $620,000 (Prop 13 base) — taxed at ~$7,200/yr.
  • New 800 sq ft ADU completed; assessor values the addition at $280,000.
  • New total assessed value: $900,000.
  • New annual tax: ~$10,440 (an increase of ~$3,250/yr).

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.

Sources

  1. LA County Assessor · Los Angeles County
  2. BOE Rule 463 — Newly Constructed Property · California State Board of Equalization
  3. Proposition 13 Letter to Assessors · California BOE
  4. California Revenue & Taxation Code §70 · California Legislature

FAQ · Finance

Common questions on finance

The questions readers send us most after this guide.

  1. What kind of return on investment can I expect?
    Across 120+ completed LA projects, our owners see 10–16% annual return on construction cost in long-term rental. The math: a $280K detached ADU renting at $2,800/month grosses $33,600/year — roughly a 12% gross yield before expenses. Property value uplift is typically 1.4–1.7× the build cost on appraisal.
  2. Are there grants or rebates available?
    The CalHFA ADU Grant Program ($40K toward soft costs) was paused in 2023 but a successor is in late-stage legislative drafting. LADWP offers rebates for high-efficiency HVAC and induction cooktops. We track every active program and apply on your behalf.
  3. What does an ADU cost in LA?
    Pricing depends on size, finish level, and site conditions. For 2026, our detached ADUs start around $250K for a 480 sq ft studio and run up to about $450K for a 1,200 sq ft custom build. Garage conversions begin around $145K. JADUs typically run $95K–$140K.
  4. What's included in your turnkey number?
    Architectural and structural design, Title 24 energy compliance, full permit fees and plan check, soils report when required, foundation, framing, MEP rough-in, insulation, drywall, doors, windows, kitchen, bathroom, flooring, paint, fixtures, and final cleaning. Excluded: landscaping beyond grading, solar (offered separately), and LADWP service upgrades when the existing panel can't carry the load.
  5. Will building an ADU raise my property taxes?
    Only the ADU portion is reassessed at its construction cost — the existing main house keeps its Prop 13 basis. On a typical $280K ADU, expect a property tax increase of roughly $2,800/year at LA's 1% rate plus local assessments.

Sources & further reading

  • California Government Code §65852.2 — statewide ADU framework (ministerial review, 60-day clock).
  • LADBS — Accessory Dwelling Unit information bulletins and current permit fee schedule.
  • HCD — California Department of Housing & Community Development, ADU handbook (2024 update).
  • Internal data: 120++ ADU projects delivered across Los Angeles County, 2018–2025.

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