Will an ADU Raise Your Property Taxes in Colorado? The Real Math

Tax Snapshot

Ask a Colorado homeowner why they have not built an accessory dwelling unit (ADU), and property taxes come up fast. Here is what the worry misses. An ADU does raise your assessment, but the increase is smaller than most people brace for, and for most owners it is dwarfed by what the unit earns. An ADU is not a tax loophole. It is also not the tax bomb it gets treated as.

• An ADU raises your taxes because the county counts the finished square footage you add. There is no version where a legal, livable unit stays invisible to the assessor.
• The size of the bump follows Colorado’s own formula, which is your added value times the residential assessment rate (about 6.8% of market value in 2026) times your local mill levy. On a typical added ADU value that lands in the hundreds of dollars a year, not the thousands.
• The offset is not close. A rentable Front Range ADU earns roughly $1,650 to $1,900 a month, which is more per month than the tax costs per year.
• Timing helps. Colorado reassesses every two years, in odd years, so the higher value does not hit your bill the day the unit is finished.
• Every figure here is directional. Your exact number depends on your county’s mill levy and your home’s value, so run your own before you decide.

How Colorado Actually Assesses Your Property

Colorado property tax runs on three numbers, and understanding them is the whole game.

Market value. The county assessor sets what your home would sell for. This is the number an ADU moves, because a finished, permitted unit adds livable space that a buyer would pay for.

Assessment rate. The state taxes only a slice of that market value, not the whole thing. For 2026 the residential rate works out to roughly 6.8% of market value, after a reduction on the first $700,000 of value. That slice is your assessed value.

Mill levy. Your local governments (school district, county, city, special districts) apply their combined mill levy to the assessed value. One mill is one dollar of tax per $1,000 of assessed value, and a typical Front Range total runs somewhere around 75 to 90 mills.

Two features of this matter for an ADU. The rate is low, so you are taxed on a small fraction of what the unit adds. And Colorado reassesses on a two-year cycle in odd-numbered years, so a change to your property shows up at the next reassessment, not the moment the crane leaves.

What an ADU Adds to the Bill

Here is the math with real numbers, so you can run your own. Say you add a 490 sq ft Olerra Flex Flat, and the finished unit lifts your home’s assessed market value by about $150,000 (a conservative figure; a detached, permitted unit often adds more). Run it through Colorado’s formula:

So a mid-range added ADU value lands you somewhere around $700 to $950 a year in additional property tax on the Front Range. Swap in your own added value and your county’s mill levy, both of which are public, and the number moves, but the order of magnitude holds at hundreds of dollars a year, not thousands. Treat this as a worked example, not a quote, and confirm the specifics with your county assessor.

The Part That Offsets It

Now set that tax against what the unit does for you, because the comparison is lopsided.

The rent alone clears the tax more than twenty times over, and that is before resale. On the value side we stay careful. Our reading of Boulder sales shows homes with an ADU sold for more, but we treat the premium as a direction and a range rather than a clean number, and we explain why in what Boulder’s sales records say about ADU value and how much an ADU adds to a Boulder home’s value. The way to hold it is simple. The tax is a small, predictable line item, and the return is the larger, if less precise, number on the other side. For the full income picture, our Colorado ADU ROI breakdown runs the rental math, and ADU financing in Colorado covers paying for the build.

Exemptions and Timing Most People Miss

A few Colorado specifics work in your favor, and owners routinely miss them.

The timing lag. Because reassessment runs every two years in odd years, the added value shows up at the next cycle, not the day you finish. Depending on when your unit is finaled, you may get most of a year before the higher bill lands.

The senior homestead exemption. If you are 65 or older and have owned and lived in the home for at least 10 years, Colorado exempts half of the first $200,000 of your home’s value from tax. Adding an ADU does not cost you that exemption on the primary residence.

Your right to protest. When the new valuation arrives, you can challenge it. Every county runs a protest window, usually in the spring, and if the assessor’s added value looks high for what you built, that is the place to say so.

Where Olerra Comes In

The tax applies cleanly only to a unit that is on the record, which is one more reason we build the way we build. Every Olerra ADU is permitted and finaled, so the value it adds, and the modest tax that follows, is legible to the assessor, a lender, and a future buyer. That is the version that appraises and earns; the tax is the small, predictable cost of doing it right. Whether you are buying a home that already has an ADU or a property you can build one on, the tax follows the same formula once the unit is on the record.

If you want to know what an ADU would do to your specific lot and your bill, a free property check is the place to start.

Frequently Asked Questions

Does an ADU raise your property taxes in Colorado?

Yes. The county assessor counts the finished square footage you add, so your assessed value, and the tax on it, goes up. A legal, livable unit cannot be invisible to the assessor. The question is not whether, it is how much, and the answer is usually smaller than owners expect.

How much will my property taxes go up with an ADU?

Run your added value through Colorado’s formula, which is added market value times the residential rate (about 6.8% in 2026) times your local mill levy (roughly 75 to 90 mills on the Front Range). For a mid-range added ADU value that lands around $700 to $950 a year. Your own number depends on your county and your home, so confirm with the assessor.

When does the higher tax actually start?

Colorado reassesses every two years, in odd-numbered years, so the added value hits at the next reassessment rather than the moment the unit is finished. You often get a lag of several months to most of a year.

Can I appeal the new valuation?

Yes. Each county has a protest window, typically in the spring, where you can challenge the assessor’s value. If the added value assigned to your ADU looks high, that is the venue to contest it.

Is the added tax worth it?

For most owners, comfortably. A rentable ADU earns more in a month than the added tax costs in a year, and a permitted unit adds resale value on top. The tax is real, but it is a small line item against what the unit returns. Confirm your own figures before you commit.

Before You Build

The honest read on ADU property taxes in Colorado is that the increase is real, modest, and predictable, and it is the smallest number in the equation once you count what the unit earns and adds.

No mystery assessment. No tax bomb. No number you cannot run yourself.

Schedule a call with Olerra or start with a free property check to see what a permitted ADU would do to your lot, your value, and your bill.

Sources

1. Colorado Division of Property Taxation, residential assessment rate and property tax procedures, 2026. dpt.colorado.gov (accessed July 2026).

2. County assessor offices (Boulder, Denver), market value, assessment, and mill levy. (accessed July 2026).

3. Colorado senior property tax (homestead) exemption, Division of Property Taxation. (accessed July 2026).

4. Olerra analysis of Boulder single-family sales (value signal) and Olerra Flex Flat rental figures. First-party, 2026.