Will an ADU Increase My Property Taxes?

A 2026 Guide for California and Los Angeles County Homeowners

By Andrea Pazmino-Pace, REALTOR®
HomeSmart Realty Group | (626) 590-1289 | AskAndreaHomes.com
Updated August 2026

Yes. Building an accessory dwelling unit, or ADU, will generally increase your property taxes in California because the completed ADU is considered assessable new construction.

However, the County Assessor normally adds only the market value created by the ADU. The existing house and land usually keep their current Proposition 13 assessed values unless another event, such as a change in ownership, requires a broader reassessment.

In simple terms, building an ADU usually creates a partial reassessment, not a full reassessment of the entire property.

Does Building an ADU Reassess the Entire Property?

Generally, no.

California’s property-tax system allows different portions of the same property to have separate base-year values.

When you build an ADU, the County Assessor typically:

1. Keeps the existing Proposition 13 value of the house and land

2. Determines how much market value the ADU added when completed

3. Establishes a new base-year value for the ADU construction

4. Adds that value to the property’s existing assessed value

The Assessor does not normally reset the entire property to its current market value solely because you built an ADU.

How Are California Property Taxes Calculated?

Under Proposition 13:

• The base property-tax rate is generally 1% of assessed value

• Local voter-approved debt can increase the total tax rate

• Existing assessed value usually increases by no more than 2% annually

• New construction can receive a separate current-market-value assessment

• A change in ownership can cause a broader reassessment unless an exclusion applies

Many Los Angeles County properties have total property-tax rates above 1% because of local bonds and voter-approved charges.

Fixed parcel taxes and direct assessments may also appear on the bill. Those charges do not necessarily change when the assessed value increases.

How Does the Assessor Value an ADU?

The Assessor determines the increment of market value the completed ADU adds to the entire property.

The assessment is not necessarily based on:

• What you paid the contractor

• The amount of your construction loan

• The ADU’s expected rental income alone

• A fixed price per square foot

• The amount you believe the ADU increased the resale value

The Assessor may consider:

• ADU size

• Construction type

• Quality and finishes

• Number of bedrooms and bathrooms

• Attached or detached design

• Garage conversion

• Utility connections

• Property location

• Local market evidence

• Completion date

• Overall contribution to property value

Construction cost and added market value can be different.

You might spend $300,000 building an ADU, but that does not automatically mean the assessed value will increase by exactly $300,000.

Example of an ADU Property-Tax Increase

Suppose your property currently has an assessed value of $450,000.

You construct a detached ADU, and the Assessor determines that it adds $250,000 in market value.

The new combined assessed value could be approximately:

Assessment component	Assessed value
Existing house and land	$450,000
New ADU value	$250,000
New total assessed value	$700,000

At the 1% base property-tax rate, the ADU portion would add approximately:

$250,000 × 1% = $2,500 per year

Local voter-approved debt could increase the final amount. Fixed charges on the tax bill may not change.

This is only an example. The Assessor determines the actual added value.

Does a Garage Conversion Increase Property Taxes?

A permitted garage conversion into an ADU can increase the assessed value, but the calculation may differ from new detached construction.

The Assessor may evaluate:

• The garage’s value before conversion

• Its value after conversion

• Structural improvements

• New plumbing and electrical systems

• Kitchen and bathroom installation

• Additional living utility

• Market value added by changing the legal use

Because the garage already existed, the added assessed value may be based on the difference between its prior use and its completed residential use.

Do not assume a garage conversion will have no tax effect because the exterior structure already existed.

Does Converting Existing Interior Space Into a JADU Increase Taxes?

A junior accessory dwelling unit, or JADU, is generally created within the walls of an existing home and has a maximum size of 500 square feet.

The tax effect depends on the actual work and whether the project creates assessable new construction.

Minor alterations or repairs may have little or no assessable effect. Major improvements that materially increase the home’s usefulness or value may create a new-construction assessment.

The County Assessor must evaluate the individual project.

When Does the Property-Tax Increase Begin?

The new assessment generally begins when the ADU is completed or available for use.

Completed new construction can generate a supplemental assessment. The supplemental bill covers the period from the first day of the month following completion through the end of the fiscal year.

California’s property-tax fiscal year runs from July 1 through June 30.

Afterward, the new ADU value normally appears on the regular annual property-tax bill.

Could You Receive More Than One Supplemental Bill?

Possibly.

Depending on the completion date and processing time, a new-construction event can result in supplemental assessments affecting more than one fiscal period.

A supplemental bill is separate from the regular secured property-tax bill.

Homeowners should not assume that paying the regular bill satisfies a later supplemental bill.

How Soon Will the Supplemental Bill Arrive?

It may not arrive immediately after the ADU is completed.

The timing depends on:

• Permit records

• Final inspection

• Information sent to the Assessor

• Appraisal workload

• Completion date

• County processing

A delayed bill does not mean the ADU escaped assessment. Set money aside for the potential supplemental bill.

Does the Assessor Receive ADU Permit Information?

Generally, yes.

Building and planning departments provide permit and construction information that can help the Assessor identify new construction.

The Assessor may also discover construction through:

• Final inspection records

• Field inspections

• Property sales

• Appraisals

• Public records

• Owner reports

• Aerial or property information

• Code-enforcement activity

According to the California State Board of Equalization, assessable construction can be valued even if it was completed without a permit.

Not obtaining permits does not prevent a property-tax assessment.

Will an Unpermitted ADU Increase Property Taxes?

It can.

An unpermitted ADU may still be assessable if the Assessor determines that it adds market value.

At the same time, an unpermitted ADU can create additional problems involving:

• Building and safety violations

• Fines and investigation fees

• Legalization costs

• Appraisal

• Financing

• Insurance

• Seller disclosures

• Rental legality

• Market value

• Future resale

Paying property taxes on an unpermitted structure does not legalize it. Tax assessment and building-code approval are separate matters.

Does the ADU Receive Its Own Tax Bill?

Usually, no.

An ADU is generally part of the same legal parcel as the primary residence. Its assessed value is added to the property’s existing assessment, and the owner receives the tax bill for the combined parcel.

Separate-conveyance rules are limited and highly specialized. Most homeowners cannot sell an ADU separately from the main property.

Will Renting the ADU Increase the Property Taxes?

Rental income alone does not usually trigger a new Proposition 13 reassessment.

The new-construction assessment results from building or converting the ADU, not simply from signing a lease.

However, renting an ADU can create other tax and financial issues involving:

• Federal and California rental income

• Depreciation

• Expense deductions

• Local registration

• Business licenses

• Rent-control rules

• Insurance

• Utility allocation

Consult a qualified tax professional before renting the unit.

Will the ADU’s Assessed Value Increase Every Year?

After the ADU receives its new base-year value, that portion of the assessment generally receives the same Proposition 13 treatment as the rest of the property.

Its annual assessed-value increase is normally limited to the lower of:

• California’s inflation adjustment

• 2%

A later change in ownership or additional new construction may create another reassessment event.

What Happens When the Property Is Sold?

When the entire property is sold, the main home, land, and ADU are generally reassessed together at the property’s current fair market value unless a valid exclusion applies.

The buyer’s new assessment is normally based on the value of the entire property, not the seller’s separate historical base-year values.

A legal, permitted ADU may contribute to the purchase price and new assessed value.

Can an ADU Increase the Home’s Market Value?

Yes, a legal and well-designed ADU may increase:

• Resale value

• Buyer demand

• Rental-income potential

• Multigenerational use

• Flexibility

• Property functionality

However, construction cost, added assessed value, and resale value are three different numbers.

For example:

• Construction cost: $300,000

• Added assessed value: $230,000

• Possible contribution to resale value: $275,000

These figures will not always match.

Market contribution depends on:

• Location

• Permits

• Size

• Condition

• Privacy

• Parking

• Layout

• Rental potential

• Utility arrangements

• Local demand

How Can You Estimate the Property-Tax Increase Before Building?

Before starting, ask the Los Angeles County Assessor how new ADU construction is generally valued.

You can also create a preliminary estimate:

1. Estimate the likely market value added by the ADU

2. Multiply that amount by the property’s approximate tax rate

3. Add the estimated increase to the current annual bill

4. Maintain a reserve for supplemental taxes

Example:

• Estimated added assessed value: $200,000

• Approximate tax rate: 1.20%

• Possible annual increase: $2,400

$200,000 × 1.20% = $2,400

This is only a planning estimate. It is not an official assessment.

Is the Property-Tax Increase Based on Rental Income?

Not necessarily.

For typical owner properties, the Assessor may focus on the market value added by the construction. Rental income could provide evidence of value, but it is not automatically multiplied by a standard amount to determine the assessment.

Different valuation methods may be considered depending on the property.

Can You Appeal the ADU Assessment?

Yes.

If you believe the Assessor assigned too much value to the completed ADU, begin by contacting the Assessor and asking for an explanation or review.

Helpful evidence may include:

• Construction plans

• Permit records

• Final inspection date

• Contractor invoices

• Photographs

• Construction-quality information

• Comparable properties with ADUs

• Professional appraisal

• Evidence of structural or utility limitations

If the disagreement is not resolved, you may be able to file a formal assessment appeal before the applicable deadline.

Continue paying the property-tax bills on time while a review or appeal is pending.

Does an ADU Affect the Homeowners’ Exemption?

An eligible owner-occupied property may still qualify for California’s Homeowners’ Exemption.

The exemption reduces the taxable assessed value by $7,000. It does not eliminate the new-construction assessment for an ADU.

If the property remains your principal residence, confirm that the exemption still appears on the tax bill.

Do ADU Repairs Increase Property Taxes?

Ordinary repairs and maintenance generally do not create a new-construction assessment when they simply restore the property to its prior condition.

Examples may include:

• Repainting

• Replacing damaged flooring

• Repairing a roof

• Replacing fixtures with comparable items

• Routine maintenance

A substantial addition, reconstruction, change in use, or major quality upgrade may be assessable.

Questions to Ask Before Building

Ask the designer, contractor, Assessor, and tax professional:

• How much market value might this ADU add?

• When will construction be considered complete?

• Could I receive supplemental tax bills?

• How will a garage conversion be valued?

• Will utility or school fees apply?

• Will the ADU affect insurance premiums?

• What rent could the unit legally generate?

• What ongoing maintenance should I expect?

• Will local rental rules apply?

• How might the ADU affect resale value?

• Can the property support the added annual taxes?

Frequently Asked Questions

Will a 500-square-foot ADU increase property taxes?

Generally, yes. The Assessor will estimate how much market value the completed unit adds.

Will a prefabricated ADU increase property taxes?

Generally, yes. The construction method does not usually eliminate the new-construction assessment.

Will a garage conversion increase taxes less than a new ADU?

Possibly. The Assessor may consider the garage’s prior value and the additional market value created by the conversion.

Is the whole house reassessed?

Generally, no. The existing home and land usually retain their Proposition 13 values. The new ADU receives a separate base-year value.

Can the ADU be taxed even without permits?

Yes. Assessment does not legalize the unit, and the unit may still create code-enforcement problems.

Will I receive a supplemental bill?

Completed new construction commonly generates a supplemental assessment before the added value appears on the regular annual bill.

Does an ADU affect capital-gains taxes?

Rental use, depreciation, and a later sale can create income-tax consequences. Consult a tax professional for advice based on your circumstances.

Conclusion

Building an ADU will generally increase property taxes in California, but it normally does not cause the entire existing property to be reassessed at current market value.

The Assessor adds the market value created by the new ADU to the property’s existing Proposition 13 assessment. The amount may be different from the construction cost or the ADU’s resale contribution.

Before building an ADU in Los Angeles, Long Beach, Baldwin Park, Downey, El Sereno, Boyle Heights, East Los Angeles, Compton, El Monte, or another Southern California community, estimate the tax increase along with construction, financing, utilities, insurance, and maintenance.

If you are considering an ADU before selling, buying, or improving a property, I can help you compare nearby homes with legal ADUs and evaluate how the unit may affect market value.

Andrea Pazmino-Pace, REALTOR®
HomeSmart Realty Group
Phone: (626) 590-1289
Website: AskAndreaHomes.com

This article provides general educational information. It is not tax, legal, appraisal, architectural, engineering, or construction advice.

 

Official sources: Los Angeles County Assessor: New Construction and ADUs, California State Board of Equalization: New Construction, California supplemental assessments, and Los Angeles County Proposition 13 information.