Self-Assessment Tax Return penalties
What happens if I miss a self-assessment tax return deadline?
If you haven't filed your tax return by the 31st of January or you've missed your July payment on account, HMRC will chase you. The penalties for failing to file your return or pay your tax are severe.
If you miss a filing deadline by one day, you're already looking at a £100 fine.
Depending on which deadline you've missed, you can expect daily penalties or fines of a percentage of the tax you owe. If HMRC thinks you're deliberately hiding something, they will investigate further and there may be serious consequences if this is the case.