March 8, 2026
By Merranti

Self-Assessment Key Dates for 2026–2027

The self-assessment system places responsibility firmly on the taxpayer. Missing a deadline — even by one day — can trigger automatic penalties, interest charges and escalating fines.

With several important dates approaching over the next year, it is crucial to understand not only when deadlines fall, but also how penalties increase the longer a return or payment remains outstanding.

Below is a practical guide to the key self-assessment deadlines for 2024/25 and 2025/26, along with the penalties that apply if you miss them.

31 January 2026 – Major Deadline for 2024/25

This is one of the most important dates in the self-assessment calendar.

What is due?

Filing deadline for 2024/25 online tax returns
Deadline for certain claims and elections for 2024/25
Payment of:
- The balance of 2024/25 tax liability
- The first payment on account for 2025/26

Penalties for late filing

If your online return is not submitted by 31 January 2026:

An automatic £100 penalty is charged
This applies even if no tax is due
For partnerships, a £100 penalty per partner applies

Paper return warning

Paper returns for 2024/25 not filed by this date will already be three months late and may attract:

Daily penalties of £10 per day
For up to 90 days
Maximum daily penalty total: £900

3 March 2026 – First Late Payment Penalty

If any 2024/25 tax remains unpaid 30 days after 31 January:

An automatic 5% late payment penalty applies to the outstanding balance.

Interest will also continue to accrue.

5 April 2026 – Four-Year Time Limit Expires

This date marks the end of the four-year time limit for certain claims and elections relating to the 2021/22 tax year.

If you need to amend or make claims for that year, action must be taken before this deadline.

30 April 2026 – Escalating Filing Penalties

At this stage:

Paper returns (2024/25)

If still not filed:

Now six months late
Further penalty of:
- 5% of tax due, or
- £300 if greater

Online returns (2024/25)

If not filed by 30 April:

Now three months late
Daily penalties of £10 per day
For up to 90 days
Maximum additional penalty: £900

31 July 2026 – Second Payment on Account

This is the due date for the second payment on account for 2025/26.

If unpaid, interest begins to accrue immediately.

Additionally:

Online 2024/25 returns

If still outstanding:

Now six months late
Further penalty:
- 5% of tax due, or
- £300 if greater

1 August 2026 – Second Late Payment Penalty

If 2024/25 tax remains unpaid six months after 31 January:

A further 5% late payment penalty is applied.

This is in addition to:

The initial 5% penalty
Ongoing interest charges

5 October 2026 – Notify Chargeability

If you have not received a tax return or notice to file but:

You have income tax or capital gains tax liability for 2025/26

You must notify HMRC of your chargeability by this date.

Failure to notify can result in additional penalties.

This commonly affects individuals who:

Start self-employment
Receive rental income
Dispose of chargeable assets
Have untaxed investment income

31 October 2026 – 2025/26 Paper Return Deadline

This is the filing deadline for 2025/26 paper tax returns.

If submitted by this date, HMRC can:

Calculate your tax for you
Confirm the amount due by 31 January 2027
Collect tax through your tax code (if under £3,000)

If filed after this date:

An automatic £100 penalty applies

2024/25 Paper Returns

If still outstanding at this point:

Now 12 months late
Further penalty:
- 5% of tax due, or
- £300 if greater

30 December 2026 – Coding Out Deadline

If you want HMRC to collect tax through your PAYE tax code (where under £3,000 is owed), your 2025/26 online return must be filed by this date.

Missing this deadline means the tax will need to be paid directly by 31 January 2027.

31 January 2027 – Next Major Deadline

This is the critical deadline for:

Filing 2025/26 online returns
Paying:
- The balancing payment for 2025/26
- The first payment on account for 2026/27

Late filing penalties and late payment penalties follow the same structure as outlined above.

How Penalties Escalate

It is important to understand how quickly penalties accumulate.

Late Filing

£100 automatic penalty after deadline
£10 per day after 3 months (up to £900)
5% or £300 (whichever is greater) at 6 months
Further 5% or £300 at 12 months

Late Payment

5% after 30 days
Additional 5% after 6 months
Additional 5% after 12 months
Interest charged throughout

Penalties can quickly exceed the original tax owed.

Time-To-Pay Arrangements (TTP)

If you are struggling to pay your tax on time, it is important not to ignore the issue.

In many cases, you can set up a Time-To-Pay arrangement with HMRC before the tax becomes overdue.

A TTP arrangement allows you to:

Spread payments over instalments
Avoid late payment penalties (if agreed in advance)
Reduce stress and uncertainty

The key is to act early. Once penalties are triggered, options become more limited.

If you anticipate difficulty meeting a deadline, please contact us as soon as possible. We can assist in:

Assessing your position
Negotiating with HMRC
Structuring a manageable payment plan

Final Thoughts

Self-assessment deadlines are strict and penalties increase significantly the longer issues remain unresolved.

The key dates to prioritise are:

31 January (filing and payment)
31 July (second payment on account)
5 October (notify chargeability)
31 October (paper returns)
30 December (coding out)

Planning ahead avoids unnecessary penalties and protects cash flow.

If you would like help preparing your return, reviewing your payments on account, or arranging a Time-To-Pay agreement, Merranti Accounting is here to help.

Proactive planning is always less expensive than reactive penalty management.