The key difference
With Self Assessment, you calculate your own tax and submit a return by 31 January. With a Simple Assessment, HMRC has already worked out what you owe from information it already holds — such as State Pension income or PAYE records that don't quite balance — and sends you a letter telling you the figure. There is nothing to fill in and no return to submit.
Who gets a Simple Assessment
GOV.UK's instruction is simple: you must pay your Simple Assessment tax bill if you've been sent a letter by HMRC. It's typically used for people whose tax affairs are straightforward enough that HMRC can calculate the figure itself, without needing a full Self Assessment return.
Paying it
You can pay online, by bank transfer or by cheque, using the 14-character payment reference starting with 'X' on your letter. GOV.UK gives these deadlines:
- Letter dated before 31 October 2026 (for the 2025/26 tax year): pay by 31 January 2027
- Letter dated on or after 31 October 2026: pay within 3 months of the date on the letter
Can you pay in instalments?
Yes — you can pay in full or in smaller instalments, as long as the whole amount is paid by the deadline. There's no need to contact HMRC first for a straightforward instalment plan within that timeframe.
What if you disagree with it?
If the figures on the letter look wrong, you can query it with HMRC rather than simply paying — but do this promptly, since the payment deadline doesn't automatically move while a query is being looked at. If your situation is more complex than the letter assumes, it's worth having it checked.
Need a hand with your return?
We prepare and file Self Assessment returns for directors, sole traders and landlords, and tell you exactly what is due and when before anything is submitted. Call 0114 327 1480.