Denver Small Business Tax Deadlines You Can't Afford to Miss
title: "Denver Small Business Tax Deadlines You Can't Afford to Miss" description: "Every federal, Colorado state, and Denver-specific tax deadline small business owners need to know — from quarterly estimates to annual returns." date: "2026-03-03" author: "Vincent Boettcher" category: "Small Business Tax"
Missing a tax deadline isn't just stressful — it's expensive. The IRS charges a failure-to-file penalty of 5% of unpaid taxes per month, up to 25%. A separate failure-to-pay penalty stacks on top at 0.5% per month. Add interest, and a missed deadline you could have managed for free becomes a bill you didn't plan for.
Denver small business owners face deadlines at three levels: federal, Colorado state, and the City and County of Denver. Here's every date that matters, in order.
Federal Tax Deadlines
March 15 — S-Corporation and partnership returns (Form 1120-S and Form 1065) are due. If your business files as an S-Corp or partnership, this is your first major deadline of the year — not April 15. Missing March 15 puts your personal return at risk too, since you need the K-1 before you can finalize your 1040.
April 15 — Individual income tax returns (Form 1040) are due. This is also the deadline for Q1 estimated tax payments (January 1 – March 31 income). If you're a sole proprietor, single-member LLC, or S-Corp shareholder who owes quarterly estimates, both your return and your first quarterly payment are due the same day.
June 16 — Q2 estimated tax payment deadline (April 1 – May 31 income). Note the slightly irregular timing — Q2 covers only two months, yet the payment is due in mid-June. Many business owners miss this one.
September 15 — Two things happen on this date. Q3 estimated tax payments are due (June 1 – August 31 income). Also, extended S-Corp and partnership returns (Forms 1120-S and 1065) are due. If you filed an extension in March, September 15 is your final deadline.
October 15 — Extended individual income tax returns (Form 1040) are due. If you filed Form 4868 in April, this is your final deadline.
January 15 (of the following year) — Q4 estimated tax payment is due (September 1 – December 31 income). This payment covers the final quarter of the prior year.
Colorado State Deadlines
Colorado generally mirrors federal deadlines for income tax purposes. Your Colorado individual income tax return (Form DR 0104) is due April 15, and your Colorado corporate/S-Corp return is due March 15. Extensions follow federal extension rules.
Colorado sales tax is a separate obligation with its own schedule:
- Monthly filers (businesses collecting over $300/month in sales tax): due on the 20th of the following month
- Quarterly filers (smaller volume): due April 20, July 20, October 20, and January 20
- Annual filers: due January 20
If your business sells taxable goods or services in Colorado, verify your filing frequency with the Colorado Department of Revenue. Incorrect frequency is a common source of missed deadlines.
Denver-Specific Obligations
The City and County of Denver has two taxes that often catch small business owners off guard.
Occupational Privilege Tax (OPT) — Denver's OPT applies to anyone who works in Denver and earns more than $500/month. As an employer, you withhold $5.75 per month per employee who meets that threshold, plus the business pays an additional $4.00 per month per qualifying employee. If you work in your own Denver-based business and pay yourself, you're both the employee and the employer for OPT purposes.
OPT returns are filed monthly and due on the last day of the following month. Annual OPT returns are also available for smaller accounts. This is a Denver-specific obligation — it doesn't apply to businesses operating only in suburban jurisdictions like Aurora, Lakewood, or Highlands Ranch.
Business Personal Property Tax — Denver (and most Colorado counties) tax tangible personal property used in a business — equipment, furniture, computers, and similar assets. The declaration schedule is typically due April 15 each year. The county assessor uses your declaration to determine your tax bill, which is then paid on the same schedule as real property taxes.
If you've never filed a business personal property tax return, check with Denver's Assessment Division. This obligation is easy to overlook, especially for new businesses.
Extension Rules: Time to File, Not to Pay
Extensions are commonly misunderstood. When you file an extension — Form 4868 for individuals, Form 7004 for business returns — you get more time to submit your return. You do not get more time to pay what you owe.
If you expect to owe taxes, you must estimate and pay that amount by the original deadline, even if you're filing an extension. Paying late, even with a valid extension on file, triggers the failure-to-pay penalty.
A properly filed extension with a reasonable payment estimate is a legitimate tool. An extension filed without a payment when you owe money is still a late payment.
Penalties at a Glance
| Situation | Penalty |
|---|---|
| Failure to file (federal) | 5% of unpaid tax per month, max 25% |
| Failure to pay (federal) | 0.5% of unpaid tax per month, max 25% |
| Both failure to file and pay | Combined up to 47.5% of unpaid tax |
| Late estimated tax payments | Underpayment penalty (calculated at IRS interest rate) |
| Colorado late filing | 5% per month, max 15% |
Interest runs on top of all federal penalties at the federal short-term rate plus 3%, compounded daily.
Frequently Asked Questions
What if I can't pay what I owe by the deadline? File your return on time regardless. The failure-to-file penalty is ten times higher than the failure-to-pay penalty. Filing on time with an unpaid balance limits your exposure — you can then set up an installment agreement with the IRS to pay over time.
Do I need to pay quarterly estimates if I'm an S-Corp owner? Yes. As an S-Corp shareholder, your distribution income isn't subject to withholding. You're responsible for making quarterly estimated tax payments on that income, plus on any salary income if withholding doesn't cover your full tax liability. Underpaying estimates results in a penalty calculated at the end of the year.
What's the deadline to elect S-Corp status for the current tax year? Generally, you must file Form 2553 within 75 days of the start of the tax year — so by March 15 for a calendar-year business. Late elections may be accepted with reasonable cause, but don't count on it.
Tax deadlines are manageable when you plan ahead. Learn more about S-Corp tax obligations — including how quarterly estimates work for S-Corp owners. Or schedule a consultation and we'll put together a personalized deadline calendar for your business.