July 29, 2026
By Merranti

What Happens If You Miss the Self Assessment Tax Return Deadline?

Missing the Self Assessment tax return deadline can be stressful.

You may have forgotten, underestimated how long it would take to gather your records or not realised that HMRC expected you to file a return.

Whatever the reason, the most important step is to act quickly.

HMRC can charge separate penalties for filing a tax return late and paying tax late. Interest may also build on unpaid tax, so ignoring the position can make it progressively more expensive.

The situation is often easier to resolve than people expect, particularly where the return is submitted and HMRC is contacted promptly.

This guide explains what happens when a Self Assessment deadline is missed, how penalties increase and what you should do next.

The short answer

If you miss the Self Assessment filing deadline, HMRC will normally charge an automatic £100 late-filing penalty.

The £100 penalty can apply even if:

You owe no tax
You already paid the tax
You are due a refund
You only missed the deadline briefly

Further penalties can arise if the return remains outstanding for three, six or 12 months.

If tax is unpaid, HMRC may also charge:

Late-payment interest
A 5% penalty after 30 days
Another 5% after six months
Another 5% after 12 months

Filing the return does not remove payment charges, but it can stop additional late-filing penalties from building.

What is Self Assessment?

Self Assessment is HMRC’s system for collecting Income Tax and other taxes that have not been fully collected automatically.

You may need to complete a tax return if you:

Are self-employed
Are a business partner
Receive rental income
Receive significant untaxed income
Have taxable overseas income
Need to report capital gains
Are liable for the High Income Child Benefit Charge
Receive income from a trust or estate
Have been told to file by HMRC

Being a company director does not automatically mean that you must submit a Self Assessment return.

A director may need to file because they receive dividends, benefits, rental income, capital gains or other income that has not been dealt with through PAYE.

Merranti’s guide to whether you need to register for Self Assessment explains the most common filing requirements.

If HMRC has issued a notice requiring you to file, you should not ignore it even if you believe no tax is due.

What are the Self Assessment deadlines?

For the 2025/26 tax year, which ended on 5 April 2026, the main deadlines are:

5 October 2026: Register if required
31 October 2026: File a paper return
30 December 2026: File online to request PAYE coding, where eligible
31 January 2027: File online and pay the balancing tax
31 January 2027: First payment on account, where applicable
31 July 2027: Second payment on account, where applicable

If HMRC issues a notice to file late, a different deadline may apply. This will normally be stated in HMRC’s letter or email.

Merranti’s guide to Self Assessment deadlines explains how the registration, filing and payment dates fit together.

What happens immediately after the deadline?

The first consequence is normally an automatic £100 late-filing penalty.

This applies even where:

The return shows no tax due
Tax was paid before the deadline
The return produces a repayment
The taxpayer misunderstood the requirement
The return is only one day late

Late-filing penalties are based on the failure to submit the return, not solely on the amount of unpaid tax.

If a partnership return is filed late, each partner can receive a separate penalty.

How do late-filing penalties increase?

The penalties become more substantial the longer the return remains outstanding.

Immediately after the deadline

HMRC normally charges:

£100 fixed penalty

More than three months late

HMRC can charge:

£10 per day
For up to 90 days
Maximum daily penalties of £900

These daily penalties are in addition to the initial £100.

More than six months late

HMRC can charge a further penalty of:

£300, or
5% of the tax due, if greater

More than 12 months late

HMRC can charge another penalty of:

£300, or
5% of the tax due, if greater

Higher penalties may apply where HMRC believes information was deliberately withheld.

How expensive can a late return become?

Assume a taxpayer files more than six months late and has no tax to pay.

The potential filing penalties could include:

Initial penalty: £100
Daily penalties: up to £900
Six-month penalty: £300
Potential total: £1,300

If the return remains outstanding for more than 12 months, a further minimum penalty of £300 may apply.

This could increase the total to £1,600, even where no tax was originally payable.

Where tax is due, percentage-based penalties may be higher.

This is why a return should be submitted promptly rather than waiting until the underlying records are perfect.

What happens if the tax is paid late?

Late-filing penalties and late-payment penalties are separate.

A taxpayer can therefore be penalised for:

Filing the return late
Paying the tax late

Late-payment penalties are generally charged at:

5% of unpaid tax after 30 days
A further 5% after six months
A further 5% after 12 months

The percentage is applied to the tax still unpaid at the relevant date.

Interest is also charged from the original payment deadline until the liability is settled.

Paying the tax does not cancel a late-filing penalty. Filing the return does not remove interest or payment penalties already arising on unpaid tax.

Will HMRC charge interest?

Yes.

HMRC normally charges late-payment interest from the day after the payment deadline until the outstanding tax is paid.

Interest is separate from:

Late-filing penalties
Late-payment penalties
Failure-to-notify penalties
Penalties for inaccurate returns

Paying part of the liability reduces the balance on which future interest is calculated.

If you cannot pay everything, making an affordable payment may therefore reduce the eventual interest cost.

Should you wait until you can afford to pay?

Generally, no.

You do not need to have the full payment available before submitting the tax return.

Filing promptly can:

Stop additional filing penalties
Establish the correct tax liability
Show whether payments on account apply
Allow a payment plan to be discussed
Prevent HMRC relying on an estimated liability

Interest and payment charges may continue on unpaid tax, but delaying the return as well as the payment usually makes the position worse.

What if you cannot afford the tax bill?

HMRC may agree to a Time to Pay arrangement, allowing the liability to be paid in monthly instalments.

HMRC will consider whether the proposed payments are affordable and whether the debt can be cleared within a reasonable period.

You may need to provide details of:

Income
Essential household spending
Savings and assets
Existing debts
The amount you can pay immediately
The monthly payment you can afford

An acceptable arrangement agreed before a late-payment penalty trigger date can prevent the corresponding penalty from arising during the arrangement.

Interest will normally continue until the tax is paid in full.

Do not wait for HMRC to begin debt-recovery action before making contact.

Can HMRC estimate the tax if you do not file?

Yes.

Where HMRC has required a return and it remains outstanding, HMRC may issue a determination estimating the tax it believes is due.

The estimate may be based on:

Previous returns
Third-party information
Earlier income levels
HMRC’s assumptions
Information already held

The determination may be higher than the actual liability.

Submitting the missing return is normally necessary to establish the correct amount, subject to the applicable legal time limits.

Ignoring a determination does not make it disappear. HMRC may pursue the estimated amount as a tax debt.

What if your records are incomplete?

Do not use incomplete records as a reason to leave the return outstanding indefinitely.

Start by gathering:

Bank statements
Sales records
Purchase invoices
Receipts
Property statements
Employment documents
Pension information
Dividend vouchers
Interest certificates
Capital disposal records

Missing records may sometimes be reconstructed using:

Online banking
Supplier copies
Customer records
Accounting software
Payment processors
Email attachments
Previous accounting reports

Where final figures genuinely cannot be established before the deadline, provisional figures may sometimes be used and clearly identified. The return can then be amended when accurate information becomes available.

This should be discussed with an accountant because estimates must have a reasonable basis and should not be presented as confirmed figures.

Merranti’s guide to records required by HMRC explains the supporting documents taxpayers should retain.

Can HMRC cancel a penalty?

HMRC may cancel a penalty where the taxpayer had a reasonable excuse.

A reasonable excuse is generally an unexpected or unusual event that prevented the person from meeting their obligation despite taking reasonable care.

Possible examples include:

Serious illness
Emergency hospital treatment
Bereavement close to the deadline
Fire, flood or theft
Unexpected computer failure
HMRC online-service problems
Severe postal disruption
Disability or mental-health difficulties
Reliance on another person in particular circumstances

Each case is considered on its facts.

The taxpayer must normally submit the return or make the payment without unreasonable delay once the problem has ended.

What will not normally be accepted?

HMRC is unlikely to accept an appeal based only on:

Forgetting the deadline
Not receiving a reminder
Finding the online system difficult
Making an avoidable mistake
Being too busy
Failing to organise records
Not having enough money
Leaving everything until January

A lack of funds may form part of a reasonable-excuse argument where it resulted from an unexpected event outside the taxpayer’s control, but financial difficulty alone is not usually sufficient.

How do you appeal a penalty?

A penalty appeal should normally explain:

Which penalty is disputed
Why the deadline was missed
When the problem began
When the problem ended
What prevented compliance
What action was taken afterwards
Why the delay was reasonable
What supporting evidence is available

Supporting evidence may include:

Medical records
Hospital letters
Death certificates
Insurance documents
Police reports
Screenshots of technical errors
Correspondence with HMRC
Evidence of postal disruption

Submitting a vague statement that the taxpayer was “busy” or “unwell” is less persuasive than explaining precisely how the circumstances prevented filing.

The outstanding return should normally be submitted before or alongside the appeal.

What if you did not need to file?

If HMRC issued a notice to file but you believe a return was not required, contact HMRC and ask for the notice to be withdrawn.

This may apply where:

Self-employment ended
Rental income stopped
Untaxed income ceased
The High Income Child Benefit Charge no longer applies
HMRC issued the return using outdated information

Do not simply ignore the notice.

Until HMRC agrees to withdraw it, the filing obligation normally remains and penalties may continue.

If HMRC confirms that the return was not required and withdraws the notice, associated late-filing penalties may also be cancelled.

What if you registered late?

Registering late can create a separate failure-to-notify penalty if the delay results in tax not being paid by the required date.

The penalty is normally based on the potential lost revenue and may depend on:

Whether tax was unpaid
How late HMRC was notified
Whether the failure was deliberate
Whether the disclosure was prompted
The quality of the disclosure

Registering after 5 October does not automatically produce a penalty where all tax is still paid on time, but the return and payment obligations remain.

What if you made a mistake on the return?

A submitted Self Assessment return can normally be amended within 12 months of the filing deadline.

For example, a return with a 31 January 2027 filing deadline can usually be amended until 31 January 2028.

Corrections may be required where:

Income was omitted
Expenses were claimed incorrectly
A figure was entered twice
A relief was missed
Bank interest was excluded
Capital gains were reported incorrectly
Payments on account were calculated incorrectly

Correcting a mistake voluntarily is normally better than waiting for HMRC to identify it.

Where the normal amendment deadline has passed, it may still be possible to write to HMRC or make an appropriate claim, depending on the circumstances.

Can an accountant submit a late return?

Yes.

An accountant can:

Confirm whether a return is required
Register you for Self Assessment
Reconstruct missing records
Prepare and submit the return
Calculate the tax due
Review payments on account
Explain penalties and interest
Prepare a penalty appeal
Assist with a Time to Pay request
Communicate with HMRC

An accountant cannot guarantee that HMRC will cancel a penalty, but a clear and properly evidenced submission can improve the quality of the response.

Common reasons taxpayers miss the deadline

Typical causes include:

Bookkeeping left until January
Missing paperwork
Forgotten HMRC login details
Delayed UTR registration
Unclear responsibility with an accountant
Undeclared rental or freelance income
Incorrect assumptions about filing
Waiting for final information
Cash-flow concerns
Last-minute software problems

Many of these issues can be prevented by starting earlier.

How to avoid future penalties

Useful habits include:

Update records monthly
Store receipts digitally
Reconcile bank accounts
Track untaxed income
Set calendar reminders
Check HMRC messages
Confirm accountant deadlines
Review payments on account
Estimate tax during the year
Keep a separate tax reserve

Regular bookkeeping can make Self Assessment preparation faster and reduce the risk of missing income, expenses or supporting records.

What should you do if your return is already late?

Take the following steps:

Confirm which return is outstanding
Check HMRC’s correspondence
Gather the available records
Submit the return promptly
Calculate the tax due
Pay as much as possible
Request Time to Pay if needed
Review the penalty notices
Appeal where a reasonable excuse exists
Correct the process for the next year

Do not wait for every issue to resolve itself before taking action.

Filing, paying and appealing are separate processes and may need to be handled at the same time.

Frequently asked questions

Will I receive a penalty if I owe no tax?

Yes.

The initial £100 late-filing penalty can apply even if no tax is due or you are entitled to a refund.

What if the return is only one day late?

The £100 penalty will normally still apply.

Should I wait until I can pay before filing?

No.

Submitting the return promptly usually prevents further filing penalties, even if the tax cannot yet be paid in full.

Can HMRC remove my penalty?

Potentially, where you have a reasonable excuse or HMRC agrees that no return was required.

Can my accountant appeal for me?

Yes.

An authorised accountant can submit an appeal and communicate with HMRC, although the taxpayer remains responsible for providing accurate information and evidence.

Can I pay the tax in instalments?

Potentially.

HMRC may agree to a Time to Pay arrangement based on affordability and your circumstances.

Will interest stop under a payment plan?

Not usually.

Interest normally continues until the outstanding tax has been paid.

What if my accountant missed the deadline?

The circumstances need to be examined carefully.

Relying on another person may sometimes support a reasonable-excuse appeal, but it does not automatically remove the taxpayer’s responsibility. Evidence of instructions, information provided and follow-up action will be important.

What if I think HMRC should not have requested a return?

Ask HMRC to withdraw the notice to file.

Do not assume that the obligation has disappeared until HMRC confirms this.

Can HMRC investigate a late return?

Yes.

Submitting a return late does not prevent HMRC from checking it. Incomplete or inconsistent returns may also prompt further questions.

Final thoughts

Missing a Self Assessment deadline is not the end of the world, but ignoring it can make the position significantly more expensive.

The best response is usually to:

File the return promptly
Establish the correct tax
Pay what you can
Contact HMRC about the balance
Appeal only where grounds exist
Improve the process for next year

The initial £100 penalty can grow into daily penalties, tax-based charges, payment penalties and interest if the return and liability remain unresolved.

Early action provides more options and demonstrates that you are trying to bring your tax affairs up to date.

Merranti Accounting can help individuals, landlords, sole traders and company directors:

Prepare outstanding returns
Reconstruct incomplete records
Calculate tax liabilities
Review HMRC penalties
Prepare reasonable-excuse appeals
Assist with payment arrangements
Correct earlier errors
Plan for future deadlines

Written by Steve Watts

Steve Watts is the Accounting Director at Merranti Accounting and a Chartered Management Accountant with the Chartered Institute of Management Accountants. He also holds a CIMA Member in Practice Certificate.

Steve qualified as a CIMA accountant in 1995 and has more than 20 years of commercial finance and accounting experience.

His career includes experience with a major housebuilding plc, the serviced-apartment sector, a large fund-owned property investment portfolio and property development. He has also worked with joint ventures, offshore companies, trusts and complex owner-managed business structures.

At Merranti Accounting, Steve supports business owners, company directors and individuals with:

Self Assessment
Tax returns
Management accounts
VAT
Payroll
Corporation Tax
Financial strategy
Business planning

Steve’s approach is based on resolving the immediate compliance issue while also identifying why it occurred.

Where a Self Assessment deadline has been missed, this means establishing which returns are outstanding, reconstructing reliable figures, calculating the amount due and ensuring that communications with HMRC are accurate and properly supported.

He also helps clients improve their record-keeping and tax-planning processes so that filing deadlines, payments on account and future liabilities can be managed earlier.

This article provides general information and should not be treated as personalised tax or legal advice. Penalties, appeals and payment arrangements depend on the taxpayer’s specific circumstances and HMRC’s assessment of the available evidence.