If you miss the deadline for filing your self-assessment tax return (31 January for online filing) you will be charged a £100 penalty.
If the return is filed more than three months late, an additional £10 per day is charged and, after six months, another penalty is incurred (the higher of £300 or 5% of the tax due). The flat-rate penalties will stand even if the tax return shows
Your company’s corporation tax return is due a year after the end of the accounting period. The penalty for being even one day late is £100 and another £100 is imposed after three months, then 10% of the tax due if the return is six months late. If you make a habit of submitting late company returns, the fixed penalties rise to £500 each time. If you have a ‘reasonable excuse’, HMRC may agree to cancel the penalty.
However, what you think is a reasonable excuse may well be found not to be so by HMRC or a tax tribunal.
Tax compliance is very important: don’t take it lightly!