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Deadlines and penalties
What happens if a client registers for Self Assessment after 5 October? The filing date moves, the payment date does not
A client who registers for Self Assessment after 5 October gets a later filing date but still pays by 31 January. The penalty rule, an example and an email.
A client who registers for Self Assessment after 5 October 2026 gets more time to file the 2025 to 2026 return, but no more time to pay. GOV.UK says HMRC will write with a filing deadline of 31 January 2027 or three months from the date on the letter, whichever is later, and that the tax is still due by 11:59pm on 31 January 2027.
The practical answer for the practice is to register the client the day they walk in, estimate the bill, and get it paid by 31 January even if the return follows in March.
Free tool
Late Self Assessment registration email maker
Fill in the client, the date you registered them and the estimated tax. The tool writes the email for the day of registration and a second for when HMRC's letter arrives, with three months from the letter's date worked out.
- The 30% in the emails is the top of the range for a failure that was not deliberate. A deliberate failure carries a higher range, so change the wording if that applies.
- Put 31 January 2027 in the diary for payment now, and add the filing date when HMRC's letter arrives.
- If three months from the date on HMRC's letter falls before 31 January 2027, the filing date is 31 January 2027.
- The emails are written for a 2025 to 2026 return. For a later year, change the tax year and the dates.
The rule this rests on: GOV.UK "Register for Self Assessment" and "Self Assessment tax returns: Penalties"; Finance Act 2008, Schedule 41. A draft to check and adapt, not legal advice.
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Three clocks
| Clock | Registered by 5 October 2026 | Registered after 5 October 2026 |
|---|---|---|
| Filing the online return | 31 January 2027 | 31 January 2027, or three months from the date on HMRC's letter if later |
| Paying the tax | 31 January 2027 | 31 January 2027 |
| Failure to notify penalty | None | Possible, on tax unpaid at 31 January 2027 |
The 5 October date comes from section 7 of the Taxes Management Act 1970, which gives six months from the end of the tax year to tell HMRC. It applies to someone who has not sent a return before, or who registered before but did not need to send a return for 2024 to 2025. The same date applies each year: 5 October 2027 for a first return for 2026 to 2027.
Section 59B of the same Act moves the payment date to three months after HMRC's notice only for a taxpayer who told HMRC in time. For everyone else the tax is due on 31 January.
The penalty is on unpaid tax
GOV.UK's penalties page says a client who registers after 5 October and does not pay all of the bill by 31 January may get a failure to notify penalty, based on the amount still left to pay. Schedule 41 to the Finance Act 2008 sets it as a percentage of the tax unpaid on 31 January following the tax year.
HMRC's factsheet CC/FS11, updated on 19 March 2026, gives the ranges:
| Behaviour | Client told HMRC before HMRC found out | HMRC found out first |
|---|---|---|
| Not deliberate, within 12 months of the tax being due | 0% to 30% | 10% to 30% |
| Not deliberate, 12 months or more after | 10% to 30% | 20% to 30% |
| Deliberate | 20% to 70% | 35% to 70% |
| Deliberate and concealed | 30% to 100% | 50% to 100% |
Where the figure lands depends on how much the client helps HMRC. There is no penalty where the client has a reasonable excuse and the failure was not deliberate.
A worked example
A client with a salary of £30,000 started a side business in June 2025 and made a profit of £4,700 by 5 April 2026. Trading income over £1,000 means a return is needed. The tax is £4,700 × 20% = £940; a profit this small attracts no Class 4 National Insurance. Because the bill is under £1,000, no payments on account are due.
The practice registers them on 20 November 2026, and HMRC's letter is dated 1 December 2026. The return is now due at the start of March 2027, on the date the letter gives. The £940 is still due on 31 January 2027.
If the client pays £940 by 31 January 2027: no tax is unpaid on that date, so there is nothing for a failure to notify penalty to be charged on, and no interest.
If the client pays £400 by 31 January and the other £540 on 26 February 2027:
- Interest runs on £540 for 26 days at HMRC's late payment rate of 7.75%: £540 × 7.75% × 26 ÷ 365 = £2.98.
- A failure to notify penalty can be charged on the £540. If the failure was not deliberate and the client came forward unprompted, the range is 0% to 30%, so between nothing and £162. If HMRC had found the client first, the range is 10% to 30%: £54 to £162.
- The payment is 26 days late, inside the 30 days after which GOV.UK's first 5% late payment penalty applies.
The return itself is not late until the date on HMRC's letter has passed. After that the usual £100 penalty applies, as our Self Assessment penalty calculator shows.
What the practice does
- Register the same day. GOV.UK's agent guidance says an agent can register a client through HMRC online services, under "Register for HMRC taxes", using a Self Assessment agent code. The alternatives are form CWF1 for a self-employed client and form SA1 for one who is not.
- Note the two dates separately. Diary 31 January 2027 for payment now. Add the filing date when HMRC's letter arrives.
- Estimate the bill early. Payment needs the client's UTR followed by the letter K as the reference, so the estimate has to be ready when the UTR arrives.
- Check whether the code can take it. GOV.UK's conditions for collecting a balance under £3,000 through a tax code do not exclude a late registrant with PAYE income, if the return is filed online by 30 December. Our guide to the 30 December filing date has the conditions.
GOV.UK gives no fixed time for a UTR to arrive; it points to HMRC's "check when you can expect a reply" tool.
An email for the late client
Subject: Your tax return: two dates, and the one that matters most
We registered you for Self Assessment on [20 November 2026]. Because that was after 5 October, HMRC will write to you with a new date for sending your return.
The date for paying has not moved. Your tax for 2025 to 2026 is due on 31 January 2027. We estimate it at [£940].
If the full amount is paid by 31 January, HMRC has no unpaid tax on which to charge a late registration penalty. If any of it is paid later, HMRC can charge a penalty of up to 30% of the unpaid part, plus interest.
Please send us [your sales and expense records] by [date] so we can firm up the figure.
Overlyft can set up an AI assistant that keeps each client's filing dates and tells the partner each morning which are at risk. Registering the client, the estimate and the advice stay with the accountant.
This is general information, not professional advice.
Short answers from this page
What happens if I register for Self Assessment after the 5 October deadline?
HMRC sends a new filing deadline: 31 January 2027 or three months from the date on its letter, whichever is later. The tax is still due on 31 January 2027. A failure to notify penalty can be charged on any tax unpaid at that date, at 0% to 30% where the failure was not deliberate.
Source: GOV.UK "Register for Self Assessment" and "Self Assessment tax returns: Penalties"; HMRC factsheet CC/FS11; read 1 October 2026
Where this comes from: GOV.UK "Register for Self Assessment", "Self Assessment tax returns" (Deadlines and Penalties), "How to register your client for a tax service as an agent" (8 September 2026), "Pay your Self Assessment tax bill", HMRC factsheet CC/FS11 "Penalties for failure to notify" (19 March 2026) and "HMRC interest rates for late and early payments"; Taxes Management Act 1970, sections 7 and 59B, and Finance Act 2008, Schedule 41, paragraph 7 (legislation.gov.uk); all read 2026-10-01.
AEO Lookup editorial team
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