A P800 is a tax calculation and HMRC will issue a P800 once a tax year has passed when they think you have overpaid or underpaid your tax. Only employees or pensioners are likely to receive a P800 because those completing tax returns should have under or over payments corrected through the self assessment system.
If you receive a P800, you should firstly check all the figures in HMRC’s calculations, by checking your wages against P60s, benefits in kind from your employer against form P11D, bank interest back to your bank statements etc. If you receive some taxable state benefits, HMRC may add them together so you might need to call them to ask for a break down of these.
Make sure HMRC haven’t missed any allowances or expenses you’re entitled to, and conversely, that they haven’t given you allowances for items you’re no longer entitled to. You don’t want another tax bill later down the line.
Once you’re happy the figures are correct, if you’re due a refund, the P800 should tell you how to go about getting your tax rebate. There are time limits for making claims for a tax refund – usually about 4 years from the end of the relevant tax year.
If you have underpaid tax of less than £3,000, HMRC will probably seek to collect it automatically from your income by adjusting your tax code. If you earn more than £30,000, HMRC could seek to collect more than £3,000 – but they can not take more than half of your wages.
If you disagree with HMRC’s calculations or you cannot afford to pay HMRC back, you should contact them as soon as possible. You might be able to spread the payments over a longer period.