31 January is a payment date, not just a filing date

If you are in Self Assessment, 31 January is three things at once: the online filing deadline for the previous tax year, the balancing payment for that year, and the first payment on account for the year you are already in. Missing the return is a £100 penalty. Missing the payment is interest, and, if you are late enough, a further penalty.

Directors of limited companies often assume PAYE and the company tax return cover them. They do not. Dividends, rental income, capital gains and the High Income Child Benefit Charge all sit on the personal return. If HMRC has issued a notice to file, you must file even if the liability is nil.

The practical habit is to treat November as the working month, not January. Records are still close to hand, and there is time to move cash. We diary personal tax work as soon as the company accounts are signed, so the two do not collide in the same week.

If you are newly in charge — a first rental property, a first dividend, or a consultancy alongside employment — you generally need to register by 5 October following the tax year. Leave that until January and you are already late before the return is even started.