If you run an agency in the UK, HMRC deadlines are not suggestions. They are hard stops. Miss one, and the penalties start stacking up fast. A late corporation tax payment can cost you thousands in interest and surcharges before you have even opened the post. A missed VAT return triggers a surcharge that compounds with each subsequent late submission.
This article covers every major HMRC deadline that matters for agency founders in the 2025/26 tax year. It is an evergreen reference post, updated annually. Bookmark it. Print it. Stick it on the wall next to your utilisation rate dashboard.
We work exclusively with agency founders at Agency Founder Finance, and these are the dates we diarise for every single client. If your current accountant has not sent you a deadlines calendar for the year ahead, you should ask why.
Corporation Tax Deadlines for Agency Companies
If your agency trades through a limited company (and most do), corporation tax is your single biggest direct tax obligation. The deadlines here are non-negotiable.
Payment deadline
Corporation tax is due nine months and one day after your company's year end. For a standard March year end (31 March 2026), that means payment is due by 1 January 2027. For a December year end (31 December 2025), payment is due by 1 October 2026.
HMRC charges interest from the day after the due date. The late payment interest rate is currently 7.25% (Bank of England base rate plus 2.5%). On a £50,000 corporation tax bill paid three months late, that is roughly £900 in interest alone. And that is before any penalty surcharges kick in.
Filing deadline
The corporation tax return (form CT600) is due 12 months after your year end. For a March 2026 year end, file by 31 March 2027. For a December 2025 year end, file by 31 December 2026.
Note the gap between payment and filing. You pay your estimated tax nine months after year end, but you have until 12 months to file the final return. If your estimate was too low, you pay interest on the underpayment from the original due date. If you overpaid, HMRC refunds with interest, but at a lower rate than they charge.
What happens if you miss it
Late filing penalties start at £100 for one day late, then escalate. After three months, another £100. After six months, HMRC estimates your tax and charges 10% of the unpaid tax. After 12 months, another 10%. These penalties apply even if you have no tax to pay.
For a typical 12-person digital agency billing £800k per year, the corporation tax bill might be £25,000 to £40,000. Missing the payment deadline by even a week creates unnecessary cost. Diarise it properly.
VAT Deadlines for Agency Founders
VAT is the deadline that catches most agency founders off guard, because it is quarterly, not annual. If you are VAT registered (compulsory above £90,000 turnover, voluntary below), you have four deadlines per year.
VAT return and payment deadlines
Your VAT quarter depends on when you registered. Typical quarters for agencies are:
- January to March: due by 7 May
- April to June: due by 7 August
- July to September: due by 7 November
- October to December: due by 7 February
You have one month and seven days after the quarter end to file and pay. If you use the annual accounting scheme, you file one return per year with payments on account throughout the year. That can simplify things for smaller agencies.
Flat rate VAT and limited cost traders
Many agencies use the flat rate VAT scheme. If you are a limited cost trader (spending less than 2% of turnover on relevant goods, or less than £1,000 per year), you revert to standard VAT accounting. HMRC has been cracking down on this. Check your position every quarter.
If you use Xero or QuickBooks, set up automatic VAT reminders in the software. Most agency founders we work with use our bookkeeping support to handle VAT returns monthly, even if they file quarterly, so there are no surprises at the deadline.
Penalties for late VAT
From January 2023, HMRC moved to a new points-based penalty system for VAT. You get one penalty point for each late submission. Reach four points (for quarterly filers) and you face a £200 penalty plus further points for each subsequent late submission. The points reset after 12 months of on-time submissions.
Late payment penalties are separate. Pay one day late, you owe 2% of the tax. Pay 15 days late, 4%. Pay 30 days or more late, 4% plus 4% per annum on the outstanding amount. These add up fast on a £15,000 VAT bill.
Payroll (RTI) Deadlines for Agency Teams
If you employ staff, you run payroll. Real Time Information (RTI) means HMRC knows what you paid before your staff do, effectively.
Monthly payroll submission
You must submit your Full Payment Submission (FPS) to HMRC on or before each payday. If you pay staff on the last Friday of the month, the FPS must reach HMRC by that Friday. No grace period. No weekend buffer.
If you miss a deadline, HMRC sends a late filing notice. Repeated late filings trigger penalties. For a 10-person agency, the penalty starts at £100 per month (per 50 employees). Small agencies are rarely hit hard on RTI penalties, but the real cost is the time spent dealing with HMRC queries.
P60 and P11D deadlines
P60s (annual earnings summaries) must be issued to employees by 31 May each year. P11Ds (benefits in kind) must be filed with HMRC by 6 July after the tax year ends. Class 1A NIC on benefits is due by 19 July (or 22 July if paid electronically).