What happens if I register as self-employed late?

What happens if I register as self-employed late? If you started working for yourself in 2025-26, you should have told HMRC by 5 October 2026. Register now: if all your 2025-26 tax is paid by 31 January 2027, there is no failure to notify penalty. No. GOV.UK says you may get a failure to notify penalty if you register after 5 October and do not pay all of your tax by 31 January. The penalty is worked out on the tax still unpaid on that date, so if everything is paid by 31 January there is nothing to base it on. It is a percentage of the tax left unpaid on 31 January after the tax year. If the failure was not deliberate the most it can be is 30%, and it can be cut to nothing if you tell HMRC yourself and HMRC knows within 12 months of the tax going unpaid. A deliberate failure can be up to 70%, or 100% if it was also concealed. An online return for 2025-26 is due by 31 January 2027, unless HMRC's notice to file it is dated after 31 October 2026. Then the law gives you 3 months from the date of the notice, and GOV.UK says HMRC's letter or email will tell you the date. Because you registered late, the tax is still due by 31 January 2027. Read the full answer, with every source, at rightotax.co.uk.