How do I appeal a Self Assessment late filing penalty?

How do I appeal a Self Assessment late filing penalty? You usually have 30 days from the date the penalty was issued to appeal, online or on form SA370, with a reasonable excuse. For 2025-26, an online return filed after 31 January 2027 gets a £100 penalty. You usually have 30 days from the date the penalty was issued. If you miss that, you can still appeal, but you will need to give a reason for being late. GOV.UK suggests you consider paying it. If the appeal is rejected and you have not paid, interest runs on the penalty from its due date. If the appeal succeeds, HMRC repays what you paid, with interest, as long as you owe no other tax. Not on its own. HMRC's manual lists pressure of work, lack of information and not getting a reminder from HMRC as things it does not normally accept by themselves. Read the full answer, with every source, at rightotax.co.uk.