Using the wrong UTR
The company’s 10-digit Corporation Tax UTR is not the same as your personal Self Assessment UTR. Returns submitted against the wrong reference will not match HMRC’s records.
Check a recent HMRC letter addressed to the company — the company UTR appears on Corporation Tax correspondence.
Missing the long-first-period rule
Many companies’ first accounts cover more than 12 months (from incorporation to the end of the first accounting year). A Corporation Tax accounting period cannot be longer than 12 months, so this requires two tax returns: one for the first 12 months, and one for the remaining days.
This surprises almost everyone filing for the first time. OneFiler flags it when your dates require it.
Confusing the payment deadline with the filing deadline
The tax payment is due 9 months and one day after your period ends — earlier than the return itself, which is due at 12 months. Filing on time but paying late still accrues interest.
Forgetting associated companies
If you control more than one company, the profit thresholds for the 19% and 25% rates are divided between them. Omitting associated companies can mean paying the wrong rate.
Mismatched dates between HMRC and Companies House
Your accounts period and your Corporation Tax accounting period usually align, but they are set by two different bodies and can drift apart — for example after changing your year end. Check both sets of dates before you file rather than assuming they match.
Leaving it until deadline day
Rejections happen — a mistyped UTR, a Government Gateway issue, a validation error. Filing with time in hand turns a rejection into an inconvenience rather than a penalty.