Blogs 01233 628648

Rapid Refund

Jump the queue, get 50% paid upfront

Average Claim £3,000

We'll tell you straight away what you're owed

Free to enquire

It's free to find out if you have a claim

No Hidden Costs

All aftercare is included

Common Tax Return Mistakes

When you’re filing a Self Assessment tax return, it’s easy to worry about getting something wrong. The good news is that most common tax return mistakes are completely fixable, and lots of people make them every year.

The key is spotting them early and knowing what to do next so you’re not hit with penalties or left paying more tax than you need to.

This guide walks you through the biggest tax return mistakes UK taxpayers make, how to avoid them, and what to do if you’ve already submitted your return and realised you’ve made an error.

Common tax return mistakes explained

Here are the mistakes HMRC sees most often, and how you can steer clear of them.

Mistake #1: Forgetting the simple details

It sounds small, but missing or incorrect information can delay your whole return. The most common slip-ups include:

Wrong National Insurance number
Out-of-date address or personal details
Missing or incorrect UTR
Mixing up PAYE employer details

These things slow HMRC down and could even trigger an enquiry. Always double-check your personal details before you file.

Mistake #2: Missing the Self Assessment deadline

The deadline to file online is 31 January each year. Miss it and you’re looking at late filing penalties, even if you don’t owe tax.

You might also face interest or extra charges if you miss the payment deadline. Payments on account catch people out too, especially if they’ve never heard of them before.

To avoid stress, file early. You’ll have more time to fix any self assessment errors before they cause problems.

Mistake #3: Entering incorrect income figures

One of the biggest mistakes on Self Assessment happens when people enter the wrong income. Common trouble spots include:

PAYE income not matching your P60 or P45
Self-employed income reported as turnover instead of profit
Missing rental income
Savings interest and dividends not declared
Side hustle earnings over the £1,000 trading allowance

An incorrect tax return can lead to penalties or underpayments. If you earn extra money from digital platforms, gig apps or selling items regularly, include it.

Mistake #4: Not declaring additional income

If money hits your bank account, HMRC wants to know about it. That includes:

Online selling or marketplace income
Freelance or contract work on the side
Rental or holiday let income
Overseas income

Leaving this off can lead to penalties based on behaviour: careless, deliberate or concealed. If you’re unsure, ask. It’s better to include income than explain why you didn’t.

Mistake #5: Forgetting allowable expenses

Lots of people miss out on money they’re owed simply by forgetting to claim their business expenses. These can include:

Tools, uniforms and protective gear
Travel to temporary workplaces
Phone and home office use
Mileage costs
Professional fees

If you don’t claim them, you’re paying tax you don’t need to. But be careful. Claiming things you’re not entitled to can also cause issues. Keep receipts and records to avoid self assessment errors.

Mistake #6: Misunderstanding tax reliefs and allowances

Missing out on allowances means missing out on savings. Common examples include:

Marriage Allowance
Trading Allowance
Personal Savings Allowance
Dividend Allowance

A lot of people also claim the wrong relief by accident. Take a moment to check which reliefs apply to you and avoid confusion later.

Mistake #7: Messy or incorrect expenses calculations

Mixing business and personal spending is a classic error. Round numbers with no records often raise questions with HMRC. Keep clear records and separate your costs where possible. It makes life a lot easier.

🖩 Free calculators: We have a range of calculators that work out everything from national insurance to rental income tax. They're free to use and provide instant estimate. No obligation required!

Tax calculators

Mistake #8: Payment on account confusion

Payments on account often surprise people. If it’s your first year filing, they might apply. If you miss them, you may build up debt without realising. This is one of the most common Self Assessment errors that leads to unexpected bills.

Mistake #9: Not checking HMRC's figures

HMRC can get things wrong too. Coding errors, incorrect PAYE figures or mismatched employer data are more common than you’d think.

If something doesn’t look right, don’t ignore it. RIFT can help you challenge HMRC mistakes and make sure you’re paying the correct amount.

Mistake #10: Leaving it all to the last minute

Last-minute tax returns often lead to missing documents, rushed figures and unnecessary errors. Give yourself time. Filing early means:

Less stress
Fewer mistakes
More time to fix problems
A better chance of catching missed refunds

Mistake #11: Not telling the whole story

HMRC penalties for tax return mistakes depend on whether the error was careless or deliberate. If you leave out income, overclaim reliefs or don’t supply accurate information, the consequences can be serious.
Fix mistakes quickly and honestly. HMRC reduces penalties when you correct issues early.

Mistake #12: Not reviewing past returns

If you’ve made mistakes in earlier years, don’t panic. You may still be able to amend them or reclaim tax you’ve overpaid. RIFT can review old returns for errors, missed expenses or opportunities to claim back money.

How to avoid tax return mistakes

Here are quick, practical ways to stay on track:

Keep records all year
Match PAYE income to your P60 or P45
Track side income from platforms and gig apps
Store receipts and note business mileage
Check your NI number, UTR and personal details
File early to avoid deadline panic
Use a Self Assessment tax return guide or let an expert handle it for you

If you’re not sure, get tax return help before submitting.

What happens if you make a mistake on your tax return?

First things first: don’t panic. Most errors are easy to fix. You can amend your return for up to 12 months after the filing deadline, and HMRC often adjusts simple errors automatically.

If the mistake means you underpaid tax, you may face penalties or interest, but correcting things early always reduces the impact. If the mistake means you overpaid, you can claim a refund.

If you’re unsure what to do, we can tell you exactly where you stand and fix the problem fast.

How to correct a Self Assessment tax return

You can amend your return online through your HMRC account. If you filed a paper return, you’ll need to make changes by post.

For older returns, you may need to apply for overpayment relief, depending on how many years have passed.

If things feel complicated or you want someone to check everything for accuracy, RIFT can handle the whole process for you.

How to correct a Self Assessment tax return

You can amend your return online through your HMRC account. If you filed a paper return, you’ll need to make changes by post.

For older returns, you may need to apply for overpayment relief, depending on how many years have passed.

If things feel complicated or you want someone to check everything for accuracy, RIFT can handle the whole process for you.

Penalties for tax return mistakes

HMRC can charge penalties for:

Missing the filing deadline
Late payment
Inaccurate information
Not declaring income
Not taking reasonable care

The amount depends on the type of mistake and how quickly you correct it. Fixing errors early always helps reduce penalties.

If you think HMRC’s figures are wrong or you’re facing charges you don’t understand, we can step in.

When HMRC make mistakes

It happens more often than you’d expect. HMRC may use incorrect PAYE data, apply the wrong tax code or miscalculate what you owe.

If you spot something odd, challenge it. Don’t assume HMRC is always right.

We can help you review your paperwork, correct errors and claim back any money you’ve overpaid.

When to get expert help

Self Assessment doesn’t have to be stressful. If you want to avoid mistakes, reduce the risk of penalties or check if you’re due a refund, RIFT’s here to help.

We’ll sort your Self Assessment correctly, fix errors, challenge HMRC when needed and help you claim back every penny you’re owed.

Let’s get your tax return right first time.

RIFTPROD1 - Subscriber

Common Tax Return Mistakes​

Common tax return mistakes explained

Mistake #1: Forgetting the simple details

Mistake #2: Missing the Self Assessment deadline

Mistake #3: Entering incorrect income figures

Mistake #4: Not declaring additional income

Mistake #5: Forgetting allowable expenses

Mistake #6: Misunderstanding tax reliefs and allowances

Mistake #7: Messy or incorrect expenses calculations

Mistake #8: Payment on account confusion

Mistake #9: Not checking HMRC's figures

Mistake #10: Leaving it all to the last minute

Mistake #11: Not telling the whole story

Mistake #12: Not reviewing past returns

How to avoid tax return mistakes

What happens if you make a mistake on your tax return?

How to correct a Self Assessment tax return

Penalties for tax return mistakes

When HMRC make mistakes

When to get expert help

Common Tax Return Mistakes