Yes, but only on the new construction. California assesses the ADU at its market value and adds that to your existing assessed value. The rest of the property keeps its Proposition 13 base. On a $250,000 ADU in Los Angeles County expect roughly $2,750 to $3,100 per year in additional property tax, against rent that commonly runs $24,000 to $36,000 a year.
- What gets assessed
- The ADU onlyHouse keeps its Prop 13 base
- LA County rate
- 1.1 to 1.25%Of the new assessed value
- On a $250k ADU
- $2,750 to $3,100 / yrAdded to your bill
- Against rent of
- $24k to $36k / yrTypical LA one-bedroom
Work out your own number. The adu cost calculator gives you a line-item range for your project in under a minute.
How the assessor finds out, and what they do
Through the building permit. Permit data goes to the county assessor, and once the permit is finaled the assessor values the new construction and adds it to your base-year value as a separate increment.
The critical point, and the one that worries owners most: your existing house is not reassessed. Proposition 13 protects the original base year value of everything that was already there. Building an ADU does not reopen your whole assessment.
The actual arithmetic
| ADU assessed at | Approximate annual tax added | Typical annual rent | Net |
|---|---|---|---|
| $150,000 | $1,650 to $1,875 | $24,000 to $30,000 | Strongly positive |
| $250,000 | $2,750 to $3,125 | $28,000 to $36,000 | Strongly positive |
| $350,000 | $3,850 to $4,375 | $33,000 to $42,000 | Strongly positive |
Los Angeles County's effective rate, including voter-approved bonds and direct assessments, generally runs between 1.1 and 1.25 percent. Your specific rate depends on your tax rate area, which is printed on your bill.
The assessed value is not the build cost
The assessor values the new construction at market value, which is not automatically what you spent. Build cost and market value usually land close on an ADU, but they are not the same number and in some markets the assessed value comes in below cost.
If you think the assessment is wrong, you can appeal it. The deadline is set by the county and the window is short, so if the notice looks high it is worth acting on immediately rather than at the next bill.
What the ADU adds back
- Rent. A one-bedroom ADU in a central or westside Los Angeles neighbourhood commonly rents in the low-to-mid $2,000s or above
- Appraised value. Appraisers increasingly treat a permitted, legal ADU as a real income-producing asset rather than a shed
- Flexibility. Family housing, a home office, or a unit you move into while renting the main house
- Buyer pool. A legal second unit widens who can buy the property, particularly to buyers who want the rent to help carry the mortgage
The property tax is real and should be in your model, but on every normal set of numbers it is a small fraction of the income the unit produces.
What does not trigger reassessment
Repairs and replacements of what already exists: a new roof, a new kitchen in the same footprint, new windows, a repipe. Those are maintenance, not new construction, and Proposition 13 protection holds.
What does trigger it: added square footage, a new bathroom, a pool, converting a garage to living space, and anything else that creates something that was not there before.
Los Angeles County figures, reviewed 2026-10-29 by America General Contractors, California contractor licence CSLB 1083262. Planning ranges, not a quote, and not legal advice. We confirm the current rules for your parcel before anything is drawn.
Questions
Will the assessor reassess my whole house?
No. Only the new construction is added. This is the single most common misunderstanding about building an ADU and it stops people who should be building.
What if I build it without a permit?
Then you have an illegal unit you cannot insure properly, cannot easily rent, and must disclose at resale, and the assessor will usually catch it eventually anyway through aerial imagery or a later permit. It is a bad trade.
Does a junior ADU get assessed?
Yes, for the value of the new construction, which on a JADU is usually small because you are mostly adding a kitchenette and a doorway rather than square footage.
Does the ADU affect my homeowner's exemption?
The homeowner's exemption applies to the property you occupy as your principal residence. Building an ADU on it does not remove the exemption, though renting out the main house and moving into the ADU can change the picture. Ask your tax adviser about your specific situation.
Can I pass the tax increase to the tenant?
In the rent, implicitly, to whatever extent the market supports. If the unit is subject to rent stabilisation, your ability to raise rent later is limited, so set the initial rent with the full carrying cost in mind.
Related reading
- Does a remodel increase property taxes in California?
- How much does an ADU cost in Los Angeles?
- Can I build an ADU on my property in Los Angeles?
