The Tax Return Deadline Isn’t Always the Mortgage Deadline

For most self employed people, 31 January is the date they associate with completing their tax return.

However, if they’re planning to buy a property, move home or remortgage, there may be another deadline to think about.

A client can be completely up to date with HMRC and still find that their latest income evidence is considered too old by a mortgage lender.

This is because some lenders apply an 18-month rule when assessing self-employed income.

What Is the 18-Month Rule for Mortgages?

Mortgage lenders need to establish that the income they’re using for affordability is current and sustainable.

For self-employed applicants, this will usually involve reviewing evidence such as:

• Finalised accounts
• Tax calculations
• Tax year overviews

Depending on the lender and how the client’s income is structured, further information may also be required.

Some lenders require the latest accounting year or tax year being used for affordability to be no more than 18 months old at the point of application.

This can create a situation where the client’s responsibilities to HMRC and the mortgage lender don’t quite line up.

The HMRC Deadline and Mortgage Deadline Can Be Different

Imagine a self-employed client who hasn’t yet completed their latest tax return.

From HMRC’s perspective, they may still have plenty of time before the filing deadline.

From a mortgage perspective, their existing income evidence could already be approaching the maximum age accepted by some lenders.

If they then decide to move home or remortgage, they could discover that the lender wants more recent figures before it will assess the application.

That doesn’t necessarily mean they’ve done anything wrong or that their tax return is late.

The lender simply has its own requirements for how recent the income evidence needs to be.

Does Every Mortgage Lender Use the 18-Month Rule?

No.

This is where it becomes important not to treat the 18-month point as a universal mortgage deadline.

Different lenders have different requirements.

Some may want the most recent figures once the previous year’s evidence reaches a certain age. Others may have different requirements or request additional information to establish whether the income remains sustainable.

The appropriate approach will therefore depend on the client’s circumstances and the lender being considered.

Why Does This Matter to Accountants?

For accountants, the important point isn’t that every self employed client needs to submit their tax return earlier.

They don’t.

The important point is knowing when a client’s future mortgage plans might make the timing relevant.

For example, if a client tells you they’re planning to:

• Buy their first home
• Move home
• Remortgage
• Raise additional borrowing

it may be worth considering their mortgage plans alongside the timing of their accounts or tax return.

A conversation between the client, accountant and mortgage adviser early in the process can help establish what information is likely to be required and when.

Should a Client Submit Their Tax Return Early for a Mortgage?

Not necessarily.

Submitting accounts or a tax return earlier purely for mortgage purposes shouldn’t be the automatic response.

First, it makes sense to establish:

• What the client wants to achieve
• When they expect to apply
• What income evidence is currently available
• Whether their latest figures are likely to be required
• Which lenders may be suitable

This is particularly important where the latest year’s income differs significantly from previous years.

The objective should be to understand the mortgage requirements before making decisions that could have wider tax or financial consequences.

Planning Ahead Can Avoid a Last-Minute Rush

The 18 month rule isn’t necessarily a problem.

It becomes a problem when nobody knows about it until the client has found a property or needs to complete a remortgage.

At that point, discovering that more recent figures are required can create unnecessary pressure for the client and their accountant.

Identifying the issue several months beforehand gives everyone more time to plan.

Final Thoughts

The tax return deadline and the mortgage deadline aren’t necessarily the same thing.

A client can be completely compliant with HMRC while their existing income evidence may no longer meet the requirements of a particular mortgage lender.

Equally, reaching the 18-month point doesn’t automatically mean a client needs to file their next tax return immediately.

Lender criteria vary.

If a self-employed client is considering a mortgage within the next six months, checking their position early can help the client, accountant and mortgage adviser understand what will be needed and avoid unnecessary last-minute work.

Information correct at time of writing – September 2026

Important Information

Your home may be repossessed if you do not keep up repayments on your mortgage or any other debts secured on it.

There may be a fee for mortgage advice. The precise amount will depend upon your circumstances.

Symmonds de Lacey is a trading style of Easy Street Financial Services Limited, which is authorised and regulated by the Financial Conduct Authority.

Please note, we are not tax advisers. For tax advice, please speak to your accountant or a suitably qualified tax adviser.