The fixed dates #
These fall on the same date for every client they apply to. All figures and dates below come from HMRC and Companies House guidance read on 19 September 2026, and the links are at the foot of the page.
| Date | What is due | If it is missed |
|---|---|---|
| 5 October | Tell HMRC a client needs to complete a tax return for the previous year | Failure to notify penalty where tax is unpaid at 31 January |
| 31 October | Paper Self Assessment return | Late filing penalty |
| 30 December | Online return, if the bill is to be collected through a tax code | Loses the coding option |
| 31 January | Online Self Assessment return, and payment of tax owed | 100 pounds, then 10 pounds a day after three months up to 900 pounds, then 5 per cent or 300 pounds at six and twelve months |
| 31 July | Second payment on account | Late payment penalties and interest |
| 19th of each month | CIS monthly return | 100 pounds at one day, 200 pounds at two months, then 300 pounds or 5 per cent of deductions |
| 6 July | P11D and P11D(b), and give employees a copy | 100 pounds per 50 employees for each month the P11D(b) is late |
| 22 July | Class 1A National Insurance, or 19 July by cheque | Interest and penalties |
| On or before payday | Full Payment Submission for payroll clients | Late reporting consequences |
| 7 Aug, 7 Nov, 7 Feb, 7 May | Making Tax Digital for Income Tax quarterly updates | Applies to each qualifying client |
The calculated dates #
These vary by client but are entirely derivable from fields a practice already holds. There is no judgement in any of them.
- Accounts at Companies House, 9 months from the end of the accounting reference period for a private company, or for first accounts the longer of 21 months from incorporation and 3 months from the accounting reference date.
- Corporation Tax payment, usually 9 months and one day after the end of the accounting period.
- Company Tax Return, 12 months after the end of the accounting period.
- Confirmation statement, at least once a year, filed up to 14 days after the review period ends.
The detail on those, including what late filing costs, is on the Companies House page.
Why this is the most mechanical work in the practice #
Look at what a reminder actually requires, and compare it with the work a practice is actually paid for.
What a reminder requires
- A date, which is either fixed or computed
- A list of clients the date applies to
- A record of who has already supplied or signed or filed
- A message
Nothing in that sequence needs an accountant, and nothing in it changes from year to year except the year.
What the practice is paid for
- Deciding what goes in a set of accounts
- Spotting that a figure is wrong
- Telling a client something they do not want to hear
Those need a qualified person and always will.
The calendar does not, and yet in most practices it is held together by a spreadsheet, a wall planner and the fact that one person has been there long enough to remember.
Count your own obligations before deciding anything #
The useful exercise is not reading a list of dates. It is multiplying.
-
Take each deadline type above, and count how many of your clients it applies to
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Add the totals
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Count how many of those need at least one reminder to happen on time, and how many need more than one
The result is the number of dated prompts your practice owes the world in a year.
Most practices have never done this sum, and the total is usually in the thousands rather than the hundreds. That is the number worth arguing about. The cost of admin time calculator will help you turn it into hours, and the January page explains why those hours are not evenly spread.
The caveats that matter #
Deadlines and penalty amounts change, so check current HMRC and Companies House guidance rather than this page, and treat nothing here as tax advice. Filing obligations rest with the client, whatever the practice does on their behalf.
And the obvious one: a reminder going out under your name is your responsibility, including whether it is accurate, whether it should have gone at all, and how the client data behind it is handled. Those are decisions a practice has to own and test.
What this page will say plainly is that working out which clients owe what on which date is arithmetic, and arithmetic is a poor use of a qualified person in January.