How long you have
GOV.UK gives you 12 months from the Self Assessment deadline to make a change, whether you are correcting a mistake, adding forgotten income, or updating a figure. For the tax year ending 5 April 2026, the filing deadline is 31 January 2027, so the amendment deadline is 31 January 2028.
Amending online
If you filed online, you need to wait 72 hours after submitting before you can make a change. Then sign in to your account, go to your Self Assessment account, choose "More Self Assessment details", then "At a glance", then "Tax return options" for the year you want to correct, and refile with the change. Your updated bill, including any interest, is worked out straight away.
Amending a paper return
Download a new SA100 (or request one from HMRC), fill in only the parts that are changing, write "amendment" on each corrected page, and send it to the address on your Self Assessment paperwork with your name and UTR clearly shown.
If the 12 months has passed
You can no longer amend the return itself. Instead you write to HMRC: to declare tax you underpaid, or to claim overpayment relief if you paid too much, which is available up to 4 years after the end of the tax year. GOV.UK is clear that this written claim needs your full details and your own signature — an accountant or agent cannot sign it on your behalf.
Correcting a return we've filed for you
If we prepared your return, tell us as soon as you spot something that needs changing. We can amend most corrections within the 12-month window quickly, and advise on an overpayment relief claim if the window has closed.
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.