Taxopedia › S › SA370S · GlossarySA370HMRC form to appeal a Self Assessment penalty. The SA370 appeals a Self Assessment penalty. Submit within 30 days of the penalty notice. Reasonable excuse arguments stand the best chance.Related termsReasonable excuseGenuine reason for missing a tax deadline — can waive penalties.Late filing penalty£100 instant fine + further penalties if your tax return is late.← Back to all terms Stop dreading tax. Start ignoring it.Get a fixed-fee quote in 60 seconds. No signup, no card, no sales call. We'll just tell you what it'll cost and how fast we can file it.Get my free quoteCall +44 7799 123045