You’ve probably heard it: “Building an ADU will double your property taxes.”
The truth? That’s a myth.

Adding an Accessory Dwelling Unit (ADU) to your California home doesn’t trigger a full property tax reassessment – but it can still increase your bill in smaller, specific ways.

Understanding how those adjustments work can help you plan smarter, avoid stress, and make the most of your investment.

At Berg Development, we believe homeowners should know exactly what to expect before breaking ground. So, let’s unpack how property taxes actually work when you add an ADU – and how to estimate your post-construction costs accurately.

How property taxes work in California (the Prop 13 basics)

To understand what happens with your ADU, you need a quick refresher on Proposition 13, the cornerstone of California property tax law since 1978.

Under Prop 13:

• Your property tax rate is capped at 1% of your property’s assessed value (plus small local add-ons).

• Your assessed value can increase by no more than 2% per year, no matter how much your home’s market value rises.

• The only time your property is fully reassessed is when there’s a change in ownership or new construction.

That last part – new construction – is what matters for ADUs.

How ADUs affect property taxes – the partial reassessment rule

Here’s the good news: when you build an ADU, the county assessor does not reassess your entire property.

Instead, only the value of the new ADU itself is added to your existing assessed value. This is called a blended or partial assessment.

Example: If your main home is assessed at $600,000 and you build an ADU valued at $150,000, your new taxable value becomes roughly $750,000.

You’ll keep your original low assessment on the main home, and only the new ADU portion will be taxed at current rates.

How assessors calculate the added value

County assessors don’t simply use your construction costs – they use market-based appraisals to estimate the ADU’s contribution to total property value.

They typically consider:

• Square footage and layout

• Construction quality and materials

• Plumbing, electrical, and HVAC additions

• Comparable ADUs recently built nearby

Once they determine the market value of the ADU, that number is added as a supplemental assessment to your tax bill.

Timing tip: This reassessment usually happens a few months after final inspection or certificate of occupancy, not right when construction starts.

Estimating your ADU’s property tax increase

Here’s a simple formula:

Added Tax ≈ (ADU value × 1%) + local county rates (typically 0.1–0.25%)

For example:

• A $150,000 ADU adds roughly $1,500 – $1,700/year to your property taxes.

• A $300,000 ADU adds about $3,000 – $3,400/year.

Still far from doubling your taxes – and often offset by rental income or property value growth.

The “replacement” rule – remodels vs. new builds

If you’re converting an existing garage or outbuilding into an ADU, things work slightly differently.

• Only the new construction portion (like plumbing, insulation, and interior finishes) is reassessed.

• The pre-existing structure’s value often stays the same.

That means garage conversions can carry smaller tax increases than new detached units.

Berg tip: Always keep documentation of what’s new versus what was existing – it helps the assessor calculate accurately.

Exemptions and tax benefits you might qualify for

California offers several programs that can ease or offset the added tax burden:

• Homeowner’s Exemption: Reduces assessed value by $7,000 if you live in the main house.

• Disabled Veterans’ Exemption: Applies if you or a family member qualifies.

• Low-income housing exemptions: In some counties, renting your ADU at affordable rates can reduce or defer property tax increases.

• Solar and energy-efficient upgrade rebates: If your ADU includes solar panels, you may qualify for federal and state credits that outweigh tax increases.

Berg stays updated on evolving ADU incentives – we help clients explore every eligible credit before breaking ground.

What about rental income and income tax?

Property taxes are one thing – income taxes are another.

If you rent out your ADU:

• You must report the rental income to the IRS.

• You can also deduct related expenses: depreciation, repairs, insurance, and a share of property taxes.

• These deductions can significantly offset taxable income.

Berg insight: Many of our ADU clients see net positive cash flow within the first year, even after accounting for added property taxes.

How Berg helps you plan ahead

We know property tax planning isn’t the most exciting part of building – but it’s one of the most important. That’s why we make it part of your financial roadmap from day one.

Our process includes:

• Detailed cost breakdowns that include estimated post-build taxes.

• Connections to local tax professionals who specialize in ADU assessments.

• ROI modeling that factors in rental potential, utility savings, and long-term appreciation.

The result: no hidden surprises – just smart planning.

Real-life examples from Berg projects

• Glendale, CA: A homeowner added a 600 sq ft ADU valued at $180,000. Their property tax rose by $1,800/year – but they now earn $2,200/month in rent.

• Burbank, CA: A garage conversion cost $120,000, adding only $1,200/year in tax, while increasing the property’s market value by over $200,000.

• Pasadena, CA: A client bundled solar and cool roofing into their ADU build, qualifying for energy rebates that offset their entire first year of added taxes.

Long-term outlook – why it’s still worth it

Even with a small tax increase, ADUs offer one of the strongest returns on investment for California homeowners:

• Rising property values = stronger home equity.

• Steady rental income offsets taxes and mortgage costs.

• Future flexibility – guest house, office, or resale appeal.

In most cases, the annual tax increase equals less than one month’s rental income, while long-term gains keep compounding.

Conclusion

Building an ADU will slightly increase your property taxes – but in exchange, you gain income potential, higher property value, and more usable space.

The key is planning ahead with accurate numbers and professional guidance.

At Berg Development, we help you understand every cost before construction begins – so your dream project builds wealth, not worry.

Ready to start your ADU with confidence? Contact Berg Development today for a consultation and personalized project estimate.