Every practice knows the shape of January: the returns that were ready in July sit untouched, and the ones missing a single bank statement consume the last week. The deadline itself is not the problem — the problem is that the work is discovered late.

This is a checklist built around when things need to start, not when they are due.

The dates that actually bind

  • 5 October — deadline to notify HMRC of chargeability for a new client who has not filed before. Miss it and a failure-to-notify penalty can follow, calculated on the tax unpaid.
  • 31 October — paper return deadline. Rarely relevant, but it matters for the small number of clients who genuinely cannot file online.
  • 30 December — the cut-off to file if the client wants a balance under £3,000 collected through their PAYE code rather than paid as a lump sum.
  • 31 January — online return, the balancing payment for the tax year just closed, and the first payment on account for the current year. All three.
  • 2 March — the 5% late payment penalty bites on anything still unpaid from 31 January.

The 30 December date is the one most often missed, and it is the one clients care about most once it is explained to them.

September and October: chase the records, not the returns

The single highest-leverage thing a practice can do in autumn is find out which clients cannot be completed, and why. A return blocked on a missing P60 in September is a five-minute email. In January it is a crisis.

Run the list and sort clients into three buckets: complete records, partial records, and no contact. Only the third group needs a phone call — the other two need a specific request for the specific missing item.

November: get the payments on account conversation done early

Clients are rarely surprised by their tax bill. They are surprised by paying it twice. A client whose liability has risen will pay the balancing payment plus 50% of the new liability on the same day in January, and the cash-flow shock is what generates the angry phone call.

Where the current year's income is genuinely lower, a claim to reduce payments on account can be made — but it carries interest if the reduction turns out to be too aggressive, so it needs a real basis, not optimism.

December: file the completed ones

Returns sitting finished but unfiled are pure risk. There is no advantage to holding a completed return back — the payment date does not move because the return was filed early, and filing releases the client's figure to them while there is still time to plan.

January: triage, don't process

By January the useful question is no longer "what can we finish?" but "what will be late, and what does that cost?" A return filed one day late attracts a £100 penalty regardless of whether tax is owed. After three months, daily penalties of £10 start accruing, up to £900.

If a return genuinely cannot be completed, filing on best estimates and amending later is usually cheaper than filing late — an amendment can be made up to 12 months after the deadline. Say so in writing to the client, and record the basis of the estimate.

Where the time actually goes

In most practices the January workload is not computation — it is reading. Bank statements, rental schedules, dividend vouchers, a shoebox of receipts, and a set of management accounts that do not tie to the previous year.

That reading is the part that can be handed off. Upload the records and ask for a summary of the income sources with anything unusual flagged, and the review time collapses to checking a set of figures that are already traced to the page they came from. TaxStats AI shows the source for every figure it gives back, which is what makes it reviewable rather than something you have to redo.

The bottom line

The practices that get through January calmly are not faster in January. They found out in September which files were incomplete. The checklist above is really one instruction repeated: discover the blocked work while there is still time for it to be unblocked.

Our UK tax deadline calendar lists every date in one place, and the deadline calculator works out the specific dates for any accounting period.