Colorado’s sales tax statute of limitations gives the Department of Revenue three years to assess additional tax, measured from the later of the date you filed the return or the date it was due. Once that window closes, the Department cannot issue a notice of deficiency, file a lien, or sue to collect for that period. The protection vanishes entirely, however, if you never filed a return or filed a fraudulent one, and several events can extend or pause the three-year clock before it runs out.
The Three-Year Rule
Under C.R.S. § 39-21-107(1), the Department must assess additional sales tax within three years after the return was filed or three years after the return was due, whichever comes later.1Justia Law. Colorado Code 39-21-107 – Limitations The Department’s own Sales Tax Guide restates the rule the same way.2Colorado Department of Revenue. Sales Tax Guide
Work it out with a concrete date. A January 2026 return due on February 20, 2026 cannot be assessed after February 20, 2029, assuming you filed on time. File the same return late, in June 2026, and the three years runs from that June filing date instead. Filing early never shortens the window; filing late lengthens it.
This three-year period covers collection as well as assessment for sales tax. No lien, distraint warrant, or collection suit can be started after it expires. The separate six-year collection window that exists elsewhere in § 39-21-107 applies to income tax, not sales tax.
When the Three Years Doesn’t Apply
Two situations strip away the limitation entirely under C.R.S. § 39-21-107(4): failing to file a return, and filing a false or fraudulent return with intent to evade tax. In either case the tax can be assessed and collected at any time.1Justia Law. Colorado Code 39-21-107 – Limitations
The sales-tax-specific statute, C.R.S. § 39-26-125, says the same thing for fraud: when a return is false or fraudulent with intent to evade, the tax, interest, and penalties may be assessed or collected without any time restriction.3FindLaw. Colorado Code 39-26-125 If you have unfiled sales tax periods, there is no year at which the Department loses the ability to assess them. Filing a complete and honest return is the only way to start the three-year clock.
Events That Extend or Pause the Clock
Even when you have filed, the three-year period is not always a hard stop.
A Proposed Adjustment Mailed Near the Deadline
If the Department sends you a written proposed adjustment before the three-year period expires, the limitation automatically extends for one year past the final determination or assessment based on that adjustment.1Justia Law. Colorado Code 39-21-107 – Limitations A proposed adjustment mailed a week before the deadline can keep your exposure alive for another full year.
Written Waiver
Before the three-year period expires, you and the Department can agree in writing to extend the assessment deadline under C.R.S. § 39-21-107(5), and the parties can sign additional extensions before any agreed-upon period runs out. The Department uses Form DR 6597 to formalize the waiver.4Colorado Department of Revenue. DR 6597 – Waiver of Statute of Limitations
Waivers usually come up during audits of complex records. Refusing can backfire. If the auditor runs out of time, the Department may issue a jeopardy assessment, a fast-tracked tax demand used when officials believe collection is at risk.5Justia Law. Colorado Code 39-21-111 – Jeopardy Assessment and Demands A jeopardy assessment is due immediately and tends to be higher than a thorough audit result, because the auditor estimates rather than verifies.
Court Control of Assets and Bankruptcy
Under C.R.S. § 39-21-107(2.5), the clock stops whenever your assets are under the control of a court, and for six months after the court releases them. It also stops during a federal bankruptcy case and for six months afterward.1Justia Law. Colorado Code 39-21-107 – Limitations
Liens Already on File
Liens filed before the three-year period expires continue for one year after the period ends, or for one year after a final determination based on a proposed adjustment, whichever applies.1Justia Law. Colorado Code 39-21-107 – Limitations The state does not lose its leverage the instant the three years run out, but only if it acted in time.
If You Get a Notice of Deficiency
Receiving a notice of deficiency means the Department beat the clock. You then have 30 days from the mailing date to request a hearing or file a written brief.6Colorado Department of Revenue. Protest Rights and Process Miss that 30-day window and the assessment becomes final.
Thirty days means 30 calendar days from the date printed on the notice, not the day you open the envelope. A timely protest triggers an informal conference where you can present records and argue your position. You do not have to pay the disputed amount first, but interest continues to accrue during the process.7FindLaw. Colorado Code 39-21-103
The Deadline Runs Both Ways: Refund Claims
If you overpaid sales tax, you must file a claim for refund within three years from the due date of the return on which the overpayment was made, or within one year of the date you overpaid, whichever is later.2Colorado Department of Revenue. Sales Tax Guide Claims must use the applicable Department form. After the deadline passes, the overpayment belongs to the state regardless of how clear the error was.
Vendors who collected too much tax from customers face an added wrinkle: depending on the circumstances, the refund claim may need to show that the excess was returned to the purchaser, or that the vendor bore the economic burden. Sorting that out in the final month is a reliable way to lose money you were entitled to recover.
Home-Rule Cities Set Their Own Timelines
The three-year rule above applies to the state-level sales tax administered by the Department of Revenue. Colorado also has dozens of home-rule municipalities, including Denver, Colorado Springs, and Aurora, that administer and collect their own sales tax. Each can set its own audit and assessment timelines, and those timelines do not necessarily match the state’s three-year window. If you collect local sales tax for a home-rule city, check that city’s municipal code for its own statute of limitations before assuming the state clock protects you.
Keeping Records Long Enough
Colorado requires retailers to keep books, accounts, and records necessary to determine the correct amount of sales tax for a minimum of three years.2Colorado Department of Revenue. Sales Tax Guide That minimum tracks the standard assessment window, but the window can stretch. A proposed adjustment mailed near the end of three years pulls you into a fourth. A signed waiver extends it further. An unfiled period never closes at all, so destroying records for a period you never reported is a gamble with no upside.
A workable practice is to keep sales tax records for at least four years after filing, which covers the three-year window plus the one-year proposed-adjustment extension. If any period is unfiled or you suspect an audit, keep everything until the matter is fully resolved.