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).
If your agency provides company cars, private medical insurance, or gym memberships, you need a P11D process. Many agencies avoid P11D entirely by using a salary sacrifice or optional remuneration arrangement. Ask your accountant which approach fits your team.
Self-Assessment Deadlines for Agency Founders
Even if your agency is a limited company, you likely file a personal self-assessment return. You pay yourself through salary and dividends, and you report those on your SA100 each year.
Online filing deadline
Self-assessment returns for the 2024/25 tax year (ending 5 April 2025) must be filed online by 31 January 2026. Paper returns must be filed by 31 October 2025.
Payment deadlines
Balancing payment for 2024/25: due 31 January 2026.
First payment on account for 2025/26: due 31 January 2026.
Second payment on account for 2025/26: due 31 July 2026.
Payments on account apply if your tax bill (excluding amounts deducted at source) is over £1,000. Each payment is half of the previous year's tax bill. If your agency dividend income drops significantly, you can apply to reduce your payments on account. Do not do this without checking with your accountant first. Underpaying triggers interest.
Late filing penalties
Day 1: £100 fixed penalty. Three months: £10 per day up to 90 days (£900 max). Six months: 5% of tax due or £300, whichever is higher. Twelve months: another 5% or £300. These escalate quickly. A £5,000 tax bill filed 18 months late can attract penalties of £1,600 or more, plus interest.
Making Tax Digital for Income Tax (MTD for ITSA)
From April 2026, Making Tax Digital for Income Tax becomes mandatory for self-employed individuals and landlords with qualifying income over £50,000 per year. From April 2027, it applies to those with income over £30,000.
If you are a sole trader agency founder or a partner in an agency partnership, you will need to use MTD-compatible software (Xero, QuickBooks, FreeAgent, Sage) to file quarterly updates to HMRC. The annual self-assessment return remains, but you submit digital updates every three months.
This is a significant change. If you are still using spreadsheets and a year-end accountant, start planning the transition now. The deadlines for quarterly updates are not yet finalised, but expect them to fall roughly one month after each quarter end.
IR35 Deadlines for Agencies Using Contractors
If your agency engages contractors through an intermediary (their limited company), you have specific IR35 obligations. Medium and large agencies are responsible for determining the contractor's employment status and issuing a Status Determination Statement (SDS) before the engagement starts.
There is no fixed HMRC deadline for issuing an SDS, but you must do it before the contractor starts work. If HMRC investigates and finds you did not issue an SDS, or issued one without taking reasonable care, you become liable for the unpaid tax and NI. That can run into tens of thousands per contractor per year.
For agencies with multiple contractors, set up a process to issue SDSs at the point of engagement. Do not backdate them. Do not use the CEST tool blindly. It is directional, not definitive. If you need help, read our guide on IR35 for agency founders.
How to Organise Your HMRC Deadlines Calendar
Here is the practical approach we recommend to every agency founder we work with:
Step 1: Map your year end and quarter ends
Your corporation tax deadline depends on your company year end. Your VAT deadlines depend on your VAT quarter. Write these down in one place. Google Calendar, Notion, a paper diary. Pick one system and stick to it.
Step 2: Set reminders 30 days before each deadline
This gives you time to gather information, check your numbers, and make payment arrangements. Most VAT and payroll issues arise because founders leave everything to the last week.
Step 3: Use your accounting software for alerts
Xero and QuickBooks both have deadline reminder features. FreeAgent sends email reminders. Turn these on. They are free and they work.
Step 4: Work with an accountant who sends you a deadlines calendar
At Agency Founder Finance, we send every client a personalised deadlines calendar at the start of the tax year. It lists their corporation tax, VAT, payroll, and self-assessment dates for the full year. No surprises. No last-minute panics.
If your current accountant does not do this, ask why. It is basic compliance hygiene.
Penalty Summary Table
| Return | Late filing penalty (typical) | Late payment interest |
|---|---|---|
| Corporation tax | £100 initial, escalating to 10%+ of tax | 7.25% per annum |
| VAT | Points-based: 4 points = £200 penalty | 2% to 4% of tax, plus 4% per annum |
| Self-assessment | £100 initial, £10/day after 3 months | 7.25% per annum |
| Payroll (RTI) | £100 per month per 50 employees | N/A (penalties only) |