What Happens If You Don’t Tell HMRC You Need to File a Tax Return?

If you discover that you should have registered for Self Assessment but did not tell HMRC by the deadline, ignoring the problem is usually the worst option.

For people who need to enter Self Assessment for the 2025/26 tax year, the deadline for notifying HMRC is generally 5 October 2026.

Missing that date does not automatically mean you will receive a penalty. But depending on whether tax is unpaid and how long the issue continues, HMRC can charge a failure-to-notify penalty, as well as interest and potentially other penalties.

Here is what you need to know.

Why do you need to tell HMRC?

The UK tax system does not always collect tax automatically.

Employees commonly have Income Tax deducted through PAYE, but HMRC may not automatically collect the correct tax on other sources of income.

You might need to tell HMRC if you have:

• Self-employed income
• Rental income
• Certain savings or investment income
• Foreign income
• Taxable capital gains
• Other untaxed income

You may also need Self Assessment because of your particular tax circumstances rather than because you run a business.

The important point is that you are responsible for checking whether you need to report something to HMRC.

What is the 5 October deadline?

If you need to complete a Self Assessment return for the 2025/26 tax year and have not previously submitted one, you generally need to notify HMRC by:

5 October 2026

The same deadline can apply if you were previously registered but did not need to submit a return for the previous tax year.

This deadline is sometimes misunderstood because it is not the date when your online tax return has to be submitted.

For 2025/26, the key dates include:

So registering and filing are two different stages.

What happens if you miss 5 October?

The first thing to do is straightforward:

Tell HMRC as soon as possible.

Do not wait until the online filing deadline simply because 5 October has already passed.

HMRC’s current guidance states that if you register after 5 October 2026, it will give you a filing deadline of three months from the date of its letter or email.

However, that does not move the normal tax-payment deadline.

Any Self Assessment tax due for 2025/26 generally still needs to be paid by 31 January 2027.

That distinction matters.

You could therefore have additional time to submit the return while still needing to calculate and pay the tax by January.

Will HMRC automatically fine you for registering late?

Not necessarily.

The penalty regime is more nuanced than simply:

“Miss 5 October = automatic fine.”

Failure-to-notify penalties are generally linked to the amount of tax that remains unpaid because HMRC was not notified on time.

If you discover the problem quickly, notify HMRC and make sure the correct tax is paid on time, the outcome can be very different from deliberately leaving undeclared tax unpaid.

The longer you leave the problem, the more difficult and potentially expensive it can become.

What is a failure-to-notify penalty?

HMRC can charge a penalty when someone fails to tell it about a tax liability when they were required to do so.

The potential penalty is generally calculated as a percentage of the potential lost revenue.

In simple terms, that means the amount of tax that HMRC could have lost because it was not told about the liability.

The percentage can depend on factors including:

• Why the failure happened
• Whether it was deliberate
• Whether information was deliberately concealed
• Whether you came forward voluntarily
• How you cooperate with HMRC

That is why there is no single fixed penalty that applies to every person who misses 5 October.

An innocent mistake and deliberately hiding income are not treated the same

HMRC’s penalty rules distinguish between different behaviours.

There is an important difference between someone who genuinely misunderstood their reporting obligations and corrects the problem promptly, and someone who knowingly conceals taxable income.

HMRC can consider whether a failure was:

• Non-deliberate
• Deliberate
• Deliberate and concealed

More serious behaviour can result in substantially more serious penalties.

If you have realised you made a mistake, dealing with it voluntarily is therefore important.

What if you genuinely did not know?

Not knowing about the requirement does not automatically remove your tax obligations.

However, the circumstances surrounding the failure can matter.

HMRC’s penalty system includes the concept of a reasonable excuse.

Whether something constitutes a reasonable excuse depends on the individual circumstances. HMRC considers what prevented you from meeting the obligation and whether you dealt with the issue without unreasonable delay once the excuse ended.

Simply saying:

“I forgot.”

or:

“I didn’t realise.”

does not automatically establish a reasonable excuse.

What if no tax was actually due?

This can make a significant difference.

Failure-to-notify penalties are connected to potential lost revenue.

So someone who registered late but had no unpaid tax is in a different position from someone who failed to declare a substantial amount of taxable income.

However, you should not assume that no tax is due without actually calculating the position.

For example, your income may be covered by an allowance or relief, but that does not necessarily mean there was no reporting obligation.

What if you started a side hustle?

This is an increasingly common situation.

Suppose you have a normal PAYE job but started freelancing during 2025/26.

Your employer deducts tax from your salary, so you assume your tax is dealt with.

But your freelance activity generated more than £1,000 of gross trading income.

HMRC generally requires sole traders with gross trading income above £1,000 to complete Self Assessment.

Your employment and your side hustle are separate for this purpose.

The fact that your salary was taxed correctly does not automatically deal with your freelance income.

What if you are a landlord?

Rental income is another area where people can unintentionally fail to notify HMRC.

You might have started receiving money from:

• A buy-to-let property
• A second property
• Renting part of a property
• Holiday accommodation
• Other property or land

The amount received, expenses, allowances and type of letting can all affect your tax position.

If you have previously undeclared rental income, establish the position rather than waiting for HMRC to contact you.

What if HMRC already knows about the income?

Do not assume that information being available to HMRC means you have fulfilled your own reporting obligations.

HMRC receives information from numerous sources and increasingly has access to financial and third-party data.

But your obligation to notify or report taxable income can still remain.

A better approach is to determine what you were required to report and make sure it has been dealt with correctly.

What if you realise several years later?

This is more serious than discovering the problem a few days after the deadline, but it can still be addressed.

You may need to establish:

• Which tax years are affected
• What income should have been declared
• What expenses or reliefs are available
• How much tax should have been paid
• Whether interest is due
• Whether penalties could apply
• How the disclosure should be made

Do not simply put several years of old income onto the current year’s tax return.

Each tax year needs to be treated correctly.

Depending on the circumstances, HMRC has specific disclosure processes that may be appropriate.

Professional advice becomes particularly valuable where undeclared income covers several years.

Will HMRC charge interest?

If tax is paid late, interest can be charged.

This is separate from penalties.

That means a taxpayer could potentially face:

Original tax liability + interest + penalties

depending on the circumstances.

The sooner an outstanding liability is identified and dealt with, the less opportunity there is for interest to continue accumulating.

What about late filing penalties?

Failure to notify HMRC and failure to submit a tax return on time are separate issues.

If HMRC requires you to submit a return and you miss the relevant filing deadline, late filing penalties can potentially apply.

That is another reason not to ignore correspondence from HMRC.

A missed registration requirement can become a much bigger problem if it is followed by:

• A late return
• Unpaid tax
• Ignored HMRC letters

Dealing with the original issue promptly can prevent several problems from stacking up.

What about late payment penalties?

Late payment is another separate issue.

For the 2025/26 Self Assessment cycle, the normal payment deadline is 31 January 2027.

If tax remains unpaid after the relevant deadlines, interest and late-payment penalties can potentially arise.

This is why someone registering late should not assume that an extended filing deadline automatically gives them extra time to pay.

What should you do if you’ve missed the deadline?

The process depends on your circumstances, but a sensible sequence is:

1. Confirm whether you actually need Self Assessment.
2. Identify which tax year or years are affected.
3. Register or reactivate your Self Assessment account as appropriate.
4. Gather your income and expense records.
5. Calculate the tax position.
6. Submit the required return or disclosure.
7. Pay any tax due as soon as required.
8. Keep evidence of what you submitted and when.

If the position covers multiple years or substantial undeclared income, get advice before trying to correct everything yourself.

Don’t wait for HMRC to find the problem

Coming forward yourself can matter when HMRC considers penalties.

HMRC distinguishes between unprompted and prompted disclosures.

Broadly, an unprompted disclosure occurs where you tell HMRC about the issue before you have reason to believe HMRC is about to discover it.

A prompted disclosure occurs after HMRC has contacted you or begun looking into the issue.

The potential penalty treatment can differ.

If you know something is wrong, voluntarily correcting it is generally preferable to waiting for an investigation.

What records should you gather?

Depending on the source of income, you might need:

• Bank statements
• Invoices
• Receipts
• Employment records
• P60s and P45s
• Rental statements
• Mortgage and property records
• Dividend information
• Savings statements
• Investment records
• Foreign income records
• Pension contribution information
• Business expense records

Do not guess at figures where reliable records are available.

Accurate records make it much easier to establish the correct tax liability.

What if you cannot afford the tax?

Do not avoid submitting a return simply because you are worried about the bill.

Filing and paying are separate obligations.

If you cannot pay everything you owe, HMRC may in some circumstances agree a Time to Pay arrangement, allowing the debt to be paid in instalments.

Whether this is available depends on your circumstances.

Ignoring the return because you cannot afford the tax can make the situation worse by adding filing penalties to an existing payment problem.

Frequently Asked Questions

What happens if I don’t register for Self Assessment by 5 October?

Register as soon as possible. Missing the deadline can potentially lead to a failure-to-notify penalty, particularly where tax remains unpaid.

Is there an automatic fine on 6 October?

Not simply because the date has passed. Failure-to-notify penalties depend on the circumstances and potential lost tax revenue.

Can I still register after 5 October?

Yes. If you discover you need Self Assessment after the deadline, you should register as soon as possible rather than waiting.

Is the tax still due on 31 January?

Generally, yes. Registering late does not automatically move the normal 31 January 2027 payment deadline for 2025/26 tax.

What if I forgot because this is my first year self-employed?

You should correct the position as soon as you realise. The circumstances of the failure and how you respond can be relevant to penalties.

What if I have undeclared income from several years?

The situation needs to be reviewed year by year. Depending on the circumstances, a specific HMRC disclosure process may be appropriate.

Should I wait to see if HMRC contacts me?

No. If you know you have a reporting obligation, address it rather than waiting for HMRC to identify the issue.

Final thoughts

Missing a tax deadline is not something to ignore, but discovering the problem does not mean you should panic either.

The key is what you do next.

If you should have told HMRC about income for 2025/26, establish the position, register where necessary and deal with the return and tax as soon as possible.

A problem identified in October can often be much easier to resolve than one left untouched for months or years.

At Merranti Accounting, we can help determine whether you need Self Assessment, establish what should have been reported and prepare the necessary return or disclosure.

Contact Merranti Accounting →

Written by Stephen Watts ACMA

Accounting Director, Merranti Accounting

Stephen is a CIMA-qualified accountant with more than 20 years of commercial finance and accounting experience. His background includes financial management, tax, business planning and supporting business owners with practical financial decisions, giving him extensive experience of helping clients understand and manage their accounting and tax obligations.

Meet Stephen and the Merranti team →