Postby someone » Tue Jan 26, 2021 11:36 am
Do you have any P11D benefits? Health insurance could well account for 250 in tax due if it's not already covered by your tax code.
Did you make more in pension contributions in previous years?
Did you make gift-aidable charitable donations in previous years?
The tax code HMRC assign you is their best guess at collecting the right tax. Sometimes they get it surprisingly accurate but other times they get it so far out it's a joke. If something has changed from the previous year and HMRC don't know yet then your tax code will be wrong.
(As one example - very large pension contribution made in one year using up all of the carry-forward available - all documented in the white space of the tax return - HMRC then provided a tax code that assumed that the same pension contribution would be made (and deductible) in the next year even though they knew that the maximum that could be made was 40K - underpaid tax would have exceeded 20K!)