Self Assessment Deadline 2027: The £100 Mistake Sole Traders Can Avoid

31 January might feel a long way away.

However, leaving your Self Assessment tax return until the last minute can quickly turn a simple job into a stressful one.

Missing the deadline can also cost you money.

HMRC can charge an initial £100 late filing penalty, even if you have little or no tax left to pay.

For sole traders, freelancers, landlords and other people who complete Self Assessment, knowing the key dates early gives you time to organise your records, understand your tax bill and prepare for payment.

This guide explains the Self Assessment deadline 2027, the penalties you could face, payments on account and the records you should prepare now.

If your bookkeeping needs organising before you start your return, Real Key Accountancy can help you get your records up to date.

What is the Self Assessment deadline for 2027?

For most people completing an online return for the 2025/26 tax year, the main deadline is:

11:59pm on 31 January 2027.

By this date, you normally need to:

• Submit your online Self Assessment tax return
• Pay any balancing tax payment you owe
• Pay your first payment on account if one applies

Therefore, 31 January can involve more than simply submitting a tax return.

You can check the official HMRC Self Assessment deadlines before filing.

Self Assessment 2026/27 key dates at a glance

These are the main dates to put in your calendar:

• 5 October 2026: Deadline to tell HMRC if you need to complete Self Assessment for 2025/26 and need to register.
• 31 October 2026: Deadline for most paper Self Assessment returns.
• 30 December 2026: Deadline if you want HMRC to consider collecting eligible Self Assessment tax through your PAYE tax code.
• 31 January 2027: Online tax return deadline.
• 31 January 2027: Deadline to pay your balancing payment.
• 31 January 2027: First payment on account may also fall due.
• 31 July 2027: Second payment on account may fall due.

As a result, preparing your return early can help you understand exactly how much money you need before January arrives.

Do you need to register by 5 October 2026?

You may need to tell HMRC that you need to complete a Self Assessment return if you have not previously registered.

For the 2025/26 tax year, HMRC says you should normally tell them by 5 October 2026 if you need to complete a return and have not already done so.

For example, this may apply if you recently became self-employed.

If you have previously used Self Assessment but did not need to submit a return last year, you may need to reactivate your account instead.

You can check the official HMRC Self Assessment registration guidance.

What happens if you miss the 31 January 2027 deadline?

Missing the deadline can become expensive.

HMRC currently applies an initial £100 late filing penalty when someone who needs to submit a return files late.

However, the penalties can increase if the return remains outstanding.

More than 3 months late

HMRC can charge additional daily penalties of £10 per day.

These can continue for up to 90 days, creating a maximum additional penalty of £900.

More than 6 months late

A further penalty can apply.

This can equal 5% of the tax due or £300, whichever is greater.

More than 12 months late

Another penalty can apply at 12 months.

Again, this can equal 5% of the tax due or £300, whichever is greater.

Therefore, ignoring a late return can make the situation much worse.

You can read the current HMRC Self Assessment penalty rules for full details.

What happens if you submit your return but pay the tax late?

Filing your return and paying your tax are two separate responsibilities.

You could submit your return on time but still face charges if you do not pay the tax by the payment deadline.

HMRC currently applies late payment penalties based on the amount of tax that remains unpaid.

Penalties can apply when tax remains unpaid after:

• 30 days
• 6 months
• 12 months

HMRC can also charge interest on overdue tax.

Therefore, knowing your tax bill before January can make budgeting much easier.

The January tax bill that catches some sole traders by surprise

One of the biggest Self Assessment surprises is not always the tax return itself.

It is payments on account.

Payments on account are advance payments towards your next Self Assessment bill.

HMRC normally asks for two payments each year when the rules apply.

• The first payment falls due on 31 January.
• The second falls due on 31 July.

Each payment is normally half of the relevant tax you owed for the previous year.

You usually do not need payments on account if your previous year’s relevant Self Assessment tax was below £1,000 or if you paid more than 80% of your tax outside Self Assessment.

HMRC explains the rules in its payments on account guidance.

Why could your January payment be higher than expected?

Imagine that this is your first year facing payments on account.

Your Self Assessment calculation shows £3,000 of relevant tax for 2025/26.

For a simple illustration, you might need to pay:

• £3,000 balancing payment for 2025/26
• £1,500 first payment on account towards 2026/27

That could create a January payment of £4,500.

You may then have another £1,500 payment on account due in July.

Your actual position can differ depending on previous payments, tax collected elsewhere and your circumstances.

However, this example shows why waiting until January to discover your bill can create cash-flow pressure.

Can you file your Self Assessment return early?

Yes.

You do not have to wait until January.

For the 2025/26 tax year, you can submit your return after the tax year ended on 5 April 2026.

Filing early does not normally mean you need to pay the tax immediately.

Instead, it can give you more time to:

• Understand your tax bill
• Check whether payments on account apply
• Budget for January
• Correct missing bookkeeping
• Find lost invoices or receipts
• Ask questions before the deadline rush

Most importantly, filing earlier can remove the January panic.

What records do sole traders need for Self Assessment?

Before starting your tax return, organise your financial records.

You may need:

• Sales income
• Customer invoices
• Business bank statements
• Purchase invoices
• Business receipts
• Mileage records
• Business expense records
• Accounting software reports
• Interest information
• Pension information where relevant
• Other taxable income details
• CIS deduction statements where relevant

Good records make the return easier to prepare.

In addition, accurate bookkeeping helps reduce the risk of forgetting legitimate business costs.

7 Self Assessment mistakes to avoid before January

1. Recording only what reached your bank

Your business records should reflect your actual income correctly.

For example, CIS subcontractors should not simply treat the net amount they receive after CIS deductions as their total sales income.

2. Forgetting small business expenses

Software subscriptions, telephone costs, mileage and other small expenses can add up throughout the year.

Therefore, check the full year rather than relying on memory.

3. Mixing personal and business spending

Mixed bank transactions make bookkeeping much harder.

A separate business account can make the process easier to manage.

4. Waiting until 31 January

January is already one of the busiest periods for accountants and tax advisers.

Starting earlier gives you more time to fix missing records and answer questions.

5. Forgetting payments on account

Your January payment may include more than the final tax from the previous year.

Therefore, check the complete HMRC calculation.

6. Guessing expenses

Do not simply estimate costs because you cannot find the paperwork.

Instead, check bank statements, emails, supplier accounts and digital receipts.

7. Assuming submission means everything is finished

After submitting your return, check how much tax you need to pay and the payment deadline.

You can find current payment options in HMRC’s Self Assessment payment guidance.

A simple Self Assessment checklist for sole traders

Use this checklist before you submit your return:

1. Confirm that you are registered for Self Assessment.
2. Check your UTR details.
3. Make sure your bookkeeping covers the full tax year.
4. Reconcile your business bank account.
5. Check all sales income.
6. Review your business expenses.
7. Find missing receipts and invoices.
8. Check your mileage records.
9. Review CIS deductions where relevant.
10. Check other taxable income.
11. Review the tax calculation.
12. Check whether payments on account apply.
13. Confirm your payment deadline.
14. Keep copies of your records.

Completing these steps early can make January much easier.

What if you cannot afford your Self Assessment tax bill?

Do not ignore the problem.

First, complete your tax return so you know how much you actually owe.

Then check the payment options available through HMRC.

HMRC allows taxpayers to make payments before the deadline, including one-off payments and regular payments towards the bill.

Some taxpayers who cannot pay in full may also qualify for additional payment arrangements.

Therefore, dealing with the issue early usually gives you more options than waiting until the debt becomes overdue.

Does Making Tax Digital change the January 2027 Self Assessment deadline?

Making Tax Digital for Income Tax started for the first group of qualifying taxpayers from 6 April 2026.

However, your 2025/26 Self Assessment return relates to the tax year before that date.

HMRC confirms that people who start using Making Tax Digital from 6 April 2026 still submit their 2025/26 Self Assessment return under the existing process by 31 January 2027.

Therefore, do not assume that joining Making Tax Digital removes the need to complete your previous tax year’s return.

When should you start preparing your 2025/26 tax return?

The best answer is simple:

As soon as your records are ready.

You gain very little by deliberately waiting until January.

Instead, preparing early gives you more time to understand your numbers and plan for the bill.

If your bookkeeping has fallen behind, start there first.

Once your income and expenses are organised, completing the tax return becomes much easier.

How Real Key Accountancy can help

Self Assessment should not become a last-minute panic.

Real Key Accountancy supports sole traders and small businesses with bookkeeping, accounting and tax-related records.

We can help you organise your bookkeeping, review the information needed for your return and identify missing records before the deadline approaches.

Most importantly, preparing earlier gives you more time to understand your tax position.

If you would like support with your records or Self Assessment, contact Real Key Accountancy to arrange a free 15-minute accounts review.

Frequently Asked Questions About the Self Assessment Deadline 2027

When is the Self Assessment deadline in 2027?

The online Self Assessment deadline for most 2025/26 tax returns is 11:59pm on 31 January 2027. You also normally need to pay any tax due by that date.

When is the paper tax return deadline?

HMRC must normally receive a paper 2025/26 Self Assessment return by 31 October 2026.

What happens if I file one day late?

If you need to submit a tax return and miss the deadline, HMRC can charge an initial £100 late filing penalty.

Do I still get a penalty if I do not owe tax?

The initial late filing penalty can still apply when a required tax return arrives late. Therefore, do not assume that having no tax to pay means you can ignore the filing deadline.

What are payments on account?

Payments on account are advance payments towards your next Self Assessment tax bill. HMRC normally asks for two instalments when the rules apply, with deadlines on 31 January and 31 July.

Can I submit my Self Assessment before January?

Yes. You can submit your 2025/26 return before January once the tax year has ended and you have the information required to complete it.

Does filing early mean I have to pay early?

Normally, filing early does not change the standard 31 January payment deadline. However, you can make payments earlier if you want to spread the cost.

What records should a sole trader keep?

You should keep suitable records of your business income, expenses, invoices, receipts, bank transactions and other information that supports your tax return.

Can Real Key Accountancy help with Self Assessment?

Real Key Accountancy can help sole traders organise their financial records and prepare for Self Assessment. Contact us to discuss the support available.

This article provides general information only and is not personalised accounting, tax or financial advice. Tax rules and individual circumstances can change, so check current HMRC guidance where necessary.