What are the penalties and interest for paying Self Assessment late?

What are the penalties and interest for paying Self Assessment late? Today, 5% of unpaid tax is added at 30 days, 6 months and 12 months, plus 7.75% yearly interest since 9 January 2026. Making Tax Digital users, and from April 2027 everyone filing a personal return, move to new penalties. 7.75% a year, in force since 9 January 2026. It is set at the Bank of England base rate plus 4%, so it changes when the base rate changes. Yes. HMRC's manual says interest arises on any tax paid late, including payments on account. Under the new Making Tax Digital penalties, payments on account do not get a late payment penalty, but interest still applies. Contact HMRC before the penalty date to agree a payment plan. HMRC's manual says no late payment penalty arises on the amount covered from the date you asked, as long as you keep to the plan. Read the full answer, with every source, at rightotax.co.uk.