The Quick Read
An accessory dwelling unit (ADU) can be the best part of a house or the most expensive surprise in it, and which one comes down to paperwork you can pull before you close. In our reading of 447 Boulder sales, two dozen homes had an ADU, and 9 of them advertised a unit the city registry had no record of. Small, directional sample, but that is better than a third of the ADUs we found sitting off the books. The listing’s word is not the record. Before you count on the unit, here is what to check.
- An ADU on the listing is not proof the ADU is legal. Confirm it in the public record before you count on it.
- Pull the property’s permit history, the final inspection or Certificate of Occupancy (CO), and the seller’s disclosure before you trust it.
- An unpermitted ADU can stall financing, fail to appraise, void an insurance claim, and become your liability the day you take title.
- Whether you can rent the unit, or must live on-site, comes down to your city’s owner-occupancy, short-term-rental, and homeowners association (HOA) rules.
- If the paperwork is not there, you still have options: renegotiate the price, walk, or budget for the legalization path, which is not always available.
What “Has an ADU” Can Actually Mean
Three very different situations all show up in a listing as the same phrase. Knowing which one you are looking at is the whole job.
Registered and finaled. The unit was permitted, inspected, and signed off. It sits in the city’s record with a Certificate of Occupancy. This is the version that appraises, insures, and rents like the asset you are paying for.
Advertised only. The listing calls it an ADU, a guest suite, a mother-in-law, or a casita, but nothing in the city record confirms it. Marketing language is not a permit. A finished basement with a kitchenette is not automatically a legal dwelling.
Unpermitted. The unit was built or converted without permits. It exists physically but not legally, and that gap becomes the new owner’s problem.
This is exactly the gap our Boulder work turned up. We walk through what the public record showed, and where it disagreed with the market, in what Boulder’s sales records say about ADU value.
This is the gap that left roughly a third of the ADUs in our Boulder sample off the registry. You cannot see it in the photos and you cannot take the listing’s word for it. The only way to know which of the three you are looking at is to pull the record.
The Documents to Pull, and What Each Proves
You can settle most of this before you write an offer, or inside your inspection window, by requesting three records from the local building department. Your agent or title company can order them. This is the checklist competitors gesture at and never actually lay out.
| Record to pull | What it proves | Where to get it | The yellow flag |
|---|---|---|---|
| Permit history | The ADU was permitted and reached a FINAL inspection, not just "issued" | Local building department, records request by address | A permit "issued" with no closeout: work never signed off |
| Certificate of Occupancy | The city says the unit is legal to occupy | Same permit file; ask for the CO or final sign-off | No CO on a finished unit: it cannot be lawfully lived in |
| Seller’s disclosure | What the seller admits about unpermitted work | Your agent or the seller (Colorado requires disclosure of known issues) | Silence or "unknown" on a unit that clearly exists |
Pull all three, not one. A permit with no Certificate of Occupancy, a CO that does not match what got built, or a disclosure that goes quiet all point the same way, and each becomes your problem the day you take title. A shed that quietly became a bedroom is the same idea we lay out in shed versus ADU: when a backyard build needs a permit.
What an Unpermitted ADU Does to Your Purchase
An unpermitted unit is not a discount version of a legal one. It is a building the record does not recognize, and that turns out to matter in five specific ways.
- Financing. Lenders underwrite against the appraisal. A buyer may not be able to borrow against space the city never approved, which can shrink what you can finance.
- Appraisal. An appraiser cannot count unpermitted square footage as living area, so the value you are paying for may not show up in the number that clears your loan.
- Insurance. A carrier can deny a claim for a loss inside a structure that was never permitted or inspected.
- Rental income. You cannot legally rent a unit the city has no record of, so any income you were counting on to justify the price can evaporate.
- Liability. It becomes yours at closing, including a possible order to bring the unit up to code or remove it at your expense.
The value side of this is real, and we quantified it: an unpermitted unit usually costs value rather than adding it. The evidence sits in our Boulder permit-record analysis, and if financing is the piece you are weighing, Colorado ADU financing breaks down the loan side. And if you are weighing the ongoing carrying cost, what an ADU does to your property taxes runs that math.
City Rules That Decide What You Can Do With It
Even a fully legal ADU comes with strings that vary by city, and they shape whether the unit does what you are buying it for.
Owner-occupancy. Some Colorado cities require you to live in either the main house or the ADU. Others dropped that rule. It decides whether you can rent both units or must occupy one.
Short-term-rental licensing. Renting the ADU on a nightly platform can require a separate license and is restricted in some cities. Denver, for example, ties short-term rental to your primary residence.
HOA covenants. Colorado’s 2024 ADU law (House Bill 24-1152) limited outright HOA bans on ADUs, but covenants can still constrain how you use one. If the property sits in an HOA, read the covenants, conditions, and restrictions before you count on a rental plan. City-by-city specifics live in our Colorado ADU rules guides.
If the ADU Isn’t Legal, Your Options
Finding an unpermitted unit does not have to kill the deal. It changes what the deal is worth and what you do next.
Renegotiate. Price the home as if the ADU is a non-conforming bonus, not a legal income unit. If you were paying a premium for a rentable second dwelling, that premium is now in question.
Walk. If the numbers only work with the ADU as legal income, and it is not, the honest move can be to pass.
Legalize it, if you can. Colorado’s Division of Local Government runs a program to help owners bring pre-existing ADUs into compliance, and some cities run their own tracks. It is not guaranteed, it can be expensive, and some units simply cannot be brought up to code. Price that risk before you take it on. Olerra builds new, permitted units and does not recertify existing ones, so a legalization project is a conversation for your city, not for us.
Where Olerra Comes In
If a house is on your list mainly because of its ADU, it is worth comparing two paths. One is buying a home with a unit whose paperwork you are still chasing. The other is buying a home you can add a permitted ADU to, one you control from day one.
Every Olerra unit is permitted and finaled, on the record where the city, the county assessor, and a future buyer’s inspector can all see it. That is the version that holds its value when you sell.
If you are weighing a specific property, a free property check will tell you what its lot and its city actually allow.
Frequently Asked Questions
How do I find out if an ADU is permitted?
Request the permit history for the address from the local building department and look for an ADU permit that reached a final inspection or a Certificate of Occupancy. A permit that was opened and never closed is a yellow flag worth chasing down before you close.
Can I get a mortgage on a house with an unpermitted ADU?
Sometimes, but you may not be able to count the ADU’s space or income toward the loan, and the appraisal may come in lower than the price. Talk to your lender early, because the answer shapes how much you can borrow.
Does the seller have to tell me the ADU is unpermitted?
In Colorado, sellers must disclose material defects they know about, and known unpermitted work is included. Read the disclosure, then verify it against the city record rather than relying on it alone.
Can I rent out the ADU after I buy?
Only if it is legal and your city allows it. Check owner-occupancy requirements and any short-term-rental licensing, plus HOA covenants if the property has them. A unit with no permit record cannot be rented lawfully.
The ADU is not permitted, but I love the house. What now?
Renegotiate the price to treat the ADU as a bonus rather than a legal unit, ask your city about the compliance path and what it would cost, and budget for the possibility that it cannot be fully legalized.
Before You Buy
An ADU is only an asset when the paperwork says so. Pull the permit history, confirm the Certificate of Occupancy, and read the disclosure before you count a single dollar of the unit toward the deal.
No guesswork. No taking the listing’s word for it. No surprises after closing.
If you would rather build a unit that is on the record from day one, schedule a call with Olerra or start with a free property check.
Sources
1. Local building and planning departments (Denver, Boulder, and Colorado Springs), permit records and Certificate of Occupancy requirements (accessed July 2026).
2. Colorado seller’s property disclosure requirement, Colorado Association of Realtors seller advisory (accessed July 2026).
3. Colorado Division of Local Government, Supporting Homeowners to Bring Pre-existing ADUs into Compliance. dlg.colorado.gov (accessed July 2026).
4. Olerra analysis of public Boulder single-family sales (ZIP 80302 and 80304) matched to the City of Boulder permitted-ADU registry and Boulder County Assessor records. First-party analysis of public records, 2026.
