Self Assessment Deadlines Loom

The normal Self Assessment filing date for online returns is 31 January after the end of the tax year. This means that 2024/25 tax returns must be filed online by 31 January 2026. The exception to this rule is if the notice to file a return was issued after 31 October 2025; where this is the case, the return must be filed no later than three months from the date on the notice to file.

However, where a client also pays tax under PAYE and wants to pay the tax year due under Self Assessment via an adjustment to their PAYE tax code, their return must be filed by the earlier date of 30 December 2025.

File by 30 December 2025 to pay tax through PAYE

Where a client has the option to pay the tax that they owe under Self Assessment through the PAYE system, this will generally be beneficial. However, this option is not open to all taxpayers with PAYE income; a client can only pay their Self Assessment bill through their tax code where all of the following apply:

• The client owes £3,000 or less through Self Assessment;
• They already pay tax under PAYE, for example because they are an employee or receive a pension; and
• The 2024/25 tax return was filed online 30 December 2025 or a paper return was filed by 31 October 2025.

Where these conditions are met, HMRC will automatically adjust the client’s tax code to collect the tax due. If the client’s return is filed by these dates and they do not want to pay their tax through PAYE, this should be made clear on the tax return.

It should be noted that the £3,000 limit applies to the total amount owed through Self-Assessment before any payments are made. Where the tax bill is more than £3,000, it is not possible to make a payment to reduce the balance to £3,000 or less and for this to be paid through PAYE.

Even if the above conditions are met, a client will not be able to pay the tax that they owe through their tax code if any of the following apply:

• They do not have sufficient PAYE income for HMRC to collect the tax that they owe;
• The client would pay more than 50% of their income in tax (the overriding regulatory limit (ORL) caps the tax that can be deducted  in a pay period at 50% of gross taxable pay; or
• The client would pay more than twice as much tax through PAYE than they would do otherwise

Where the PAYE payment route is taken, the client’s 2026/27 tax code will be adjusted to collect the tax that the client owes. For example, if the client owes £2,000 and says tax at 40%, their tax code will be reduced by £5,000 times collect the tax that they owe (40% of £5,000 = £2,000). The tax will be paid in equal instalments throughout the 2026/27 tax year in accordance with the client’s pay frequency.

Paying a Self Assessment bill through PAYE offers a number of advantages. Where this route is taken, the tax is paid later, which offers cash flow benefits. Instead of settling the balance by 31 January 2026, the tax will be paid in instalments during the 2026/27 tax year.

Paying tax through an adjustment to the PAYE code provides an automatic instalment payment option- the client does not need to set up a Time to Pay Arrangement to pay an instalments and unlike a Time to Pay arrangement, the instalments are interest free. This is a big plus.

Paying through their PAYE code is likely to be of interest to employed clients who have a small amount tax due under Self Assessment, for example on income from a side hustle or on rental income. It is important that they are made aware of the need to file their return by 30 December to benefit from this.

Paying tax through PAYE will not be the preferred option for everyone as it will reduce their take-home pay. Paying the bill in full by 31 January in order to maintain their monthly income may be a better option for some clients.

31 January 2026

The main deadline in the Self Assessment calendar is 31 January after the end of the tax year. This is the date by which tax returns must be filed online if a late filing penalty is to be avoided (unless a later deadline applies because the notice to file was issued after 31 October 2025). If the deadline is missed, the client will automatically receive a late filing penalty of £100, even if they have no tax to pay for 2024/25.

Any tax and Class 4 National Insurance owing for 2024/25 must also be paid by January 2026, unless the client has set up a Time to Pay Arrangement or filed their return online by 30 December 2025 (or on paper by 31 October 2025) and is paying their bill through PAYE.

Where the client’s tax and Class 4 NIC bill for 2024/25 is £1,000 or more, they will need to make payments on account towards their 2025/26 liability, unless 80% of their tax bill for the year is collected at source, such as through PAYE.

Each payment on account is 50% of the tax and Class 4 National Insurance bill for the previous tax year. For example, if a client owes tax and Class 4 National Insurance of £8,240 for 2024/25, they will need to make two payments on account of their 2025/26 liability of £4,120. The first payment on account for 2025/26 must be paid on 31 January 2026 (together with any remaining tax and Class 4 National Insurance due for 2024/25). The second payment on account is due by 31 July 2026. Any balance that remains due must be paid by 31 January 2027.

If a client did not make payments on accounts for 2024/25 but their tax and Class 4 National Insurance bill for that year is more than £1,000 on 31 January 2026. they will need to make a payment equal to 150% of their 2024/25 liability, comprising the tax and Class 4 National Insurance that they owe for 2024/25 and their first payment on account for 2025/26 (being 50% of their 2024/25 bill). This may be significant and clients affected may wish to consider ways of spreading the cost.

When calculating 2024/25 bills, remember that some clients may also have tax to pay on transition profits as a result of the move from the currently year basis to the tax year basis.