Payroll feels straightforward until it isn't. You hire a few staff, calculate their pay, deduct tax and National Insurance, and send money to HMRC. Simple enough — in theory. In practice, payroll is one of the most regulation-heavy tasks a small business owner faces, and the margin for error is narrower than most people realise. Miss a deadline, miscalculate a statutory payment, or submit the wrong figures, and HMRC will notice. When they do, the penalties can escalate quickly and quietly, often long before you spot the problem yourself.
This guide cuts through the jargon to show you exactly what HMRC can charge, which mistakes trigger the highest penalties, and what you can do right now to make sure you're not caught out.
How HMRC's RTI System Catches Errors in Real Time
Since April 2013, employers have been required to report PAYE information to HMRC in real time — this is known as Real Time Information, or RTI. Every time you pay an employee, you must submit a Full Payment Submission (FPS) on or before the payment date. If you make no payments in a tax month, you may need to submit an Employer Payment Summary (EPS) instead.
The RTI system means HMRC receives payroll data almost simultaneously with your employees. There is no waiting until year-end to reconcile figures. This is useful for employees claiming Universal Credit, but it also means HMRC can identify discrepancies — and begin accumulating penalty charges — far sooner than under the old annual P35 regime.
If your FPS is late, HMRC's system flags it automatically. You don't receive a polite warning first. The penalty clock starts ticking.
The HMRC Penalty Structure: What You'll Actually Pay
Understanding the penalty structure is the first step to appreciating the real financial risk. HMRC applies penalties for late FPS submissions in monthly bands based on the number of employees on your payroll:
- 1–9 employees: £100 per month
- 10–49 employees: £200 per month
- 50–249 employees: £300 per month
- 250 or more employees: £400 per month
These charges apply for each tax month in which you submit late. Fail to file for three consecutive months and HMRC can issue an additional penalty of 5% of the tax and National Insurance that should have been reported. That percentage escalates further if the failure continues.
Beyond late filing, there are penalties for inaccuracies in the figures themselves. If HMRC determines that an error led to an underpayment of tax or NI, they will assess the unpaid amount plus interest at their current rate (currently 7.75% per annum for late payment). On top of that, penalty surcharges apply:
- Careless errors: 0%–30% of the unpaid tax (potentially reduced to zero if you disclose unprompted)
- Deliberate errors: 20%–70%
- Deliberate and concealed errors: 30%–100%
HMRC assesses intent based on the evidence, so a genuine mistake that you disclose proactively will almost always attract a far lower penalty than one discovered during an enquiry. Transparency matters.
Common Payroll Mistakes That Trigger Penalties
Knowing where errors cluster helps you focus your attention. The following are among the most frequent payroll mistakes HMRC encounters with small UK employers:
- Using the wrong tax code. An employee starting without a P45 should be placed on a starter checklist to determine the correct emergency code. Using the wrong code from day one can mean underpaying or overpaying PAYE tax — both create reconciliation problems.
- Incorrect National Insurance category letters. Different employees attract different NI rates. Veterans under 21, apprentices under 25, and employees over State Pension age all have specific category letters. Using category A for everyone is a common and costly mistake.
- Miscalculating Statutory Sick Pay (SSP) or Statutory Maternity Pay (SMP). These have specific qualifying rules, waiting days, and rate calculations. Errors here not only affect employees' take-home pay but can result in HMRC recovering overclaimed relief.
- Missing the FPS deadline. Even being one day late counts. If payday falls on a weekend or bank holiday, the submission must still be made on or before the contractual payment date — not the next working day.
- Failing to register as an employer before the first payday. HMRC requires registration before you pay your first employee. Many new employers discover this rule only after their first payroll run, which immediately creates a late submission.
Auto-Enrolment: A Separate Penalty Regime You Can't Ignore
HMRC penalties cover PAYE and NI, but auto-enrolment failures fall under The Pensions Regulator (TPR) — a completely separate body with its own enforcement powers. If you employ even one eligible worker aged between 22 and State Pension age earning more than £10,000 per year, you have auto-enrolment duties.
TPR's fixed penalty for non-compliance is £400. If the breach continues after that notice, escalating daily penalties apply: £50 per day for employers with 1–4 staff, rising to £500 per day for those with 50–249 employees. These are not hypothetical figures — TPR issued over 50,000 compliance notices in a single recent year.
You must also re-enrol eligible workers every three years and submit a re-declaration of compliance to TPR. Missing the re-enrolment window is an easy oversight with serious consequences, particularly for sole traders who take on their first members of staff and are unfamiliar with the process.
How to Protect Your Business from Payroll Penalties
The good news is that most payroll penalties are entirely preventable. Here is what genuinely makes a difference:
- Use compliant payroll software that connects directly to HMRC. Software that submits FPS and EPS directly via HMRC's RTI API removes the risk of manual submission errors and late filing. BizHub365, for instance, handles RTI submissions natively — no bridging software, no copying figures between systems — and generates P60s and P45s automatically when needed.
- Set up calendar reminders for every payroll deadline. Your FPS must reach HMRC on or before each payday. Build in a day's buffer and treat it as a hard deadline, not an approximation.
- Keep employee records scrupulously up to date. A change of address has no penalty implications, but a change in hours, salary, or NI category that isn't reflected in your next FPS absolutely does.
- Conduct a payroll audit at least once a year. Compare your payroll records against what HMRC holds on your PAYE Online account. Discrepancies caught internally cost far less to fix than those uncovered during an HMRC enquiry.
- If you spot a mistake, disclose it promptly. HMRC's penalty reduction scheme rewards unprompted disclosure. A voluntary correction will almost always result in a smaller penalty than waiting for HMRC to find the error themselves.
Conclusion: The Cost of Complacency
Payroll errors rarely feel urgent in the moment. A miscalculated NI contribution or a day-late FPS submission seems minor — until the penalties compound and HMRC sends a notice of determination. At that point, you are dealing with back payments, interest charges, penalty surcharges, and the administrative burden of responding to an enquiry, often while trying to run a business at the same time.
The best payroll strategy is a proactive one. Understand the rules, meet every deadline, and use software that does the heavy lifting for you. For UK small business owners who want one less thing to worry about, getting payroll right from day one is not just good practice — it is the most cost-effective decision you will make.