Common Tax Return Mistakes​

Mistake #1: Forgetting to check the simple details

These may sound obvious, but it's often the simplest details that trip people up. You might have calculated every last penny of every last expense. You might have kept and organised all your bank statements and payslips. However, all that care and effort mean nothing if you haven't done the following:

• Sign and date your paper tax return
• Returned the Authorising your agent (64-8) form to let your accountant act as your agent
• Remembered to check such things as whether you have to use the original form or if a scan or photocopy is ok, or whether the taxman needs a physical signature.
  
  RIFT tip: Filing online might be a smart move to avoid easy mistakes like this. Get self-assessment tax return help with RIFT, and you don't have to worry.

Mistake #2: Skimping on details

What HMRC want from your self-assessment tax return is a complete overview. They won't be satisfied with anything less. Skipping over things you aren't sure of, or writing "details to follow" will only land you in hot water. Always use cold, hard numbers when they're asked for.

RIFT tip: Remember you can save your progress on the returns website. If you're waiting for exact figures or details, you can always come back and fill them in later. Just remember to get it all finalised before the deadline.

Mistake #3: Entering incorrect income figures 

Incorrect income reporting is a major tax return error, whether you’re self-employed or on PAYE. Mistakes here can result in overpaying tax or triggering an investigation by HMRC.

Self-employed earnings

• Report your total earnings accurately, including any work outside your main business.
• Keep clear records of all invoices and payments received.

PAYE errors

• If you have multiple jobs or receive benefits like company bonuses, double-check your P60 or P45.
• HMRC often updates tax codes incorrectly, leading to miscalculations.

RIFT tip: The website forms will do quite a lot of the actual calculations for you. Again, though - if you're even slightly concerned about your accuracy then professional advice is the way to go. Use our self-employed tax calculator to work out your earnings.

Mistake #4: Missing the tax return deadline

One of the most common self-assessment mistakes is failing to file on time. HMRC has strict deadlines and missing them can result in automatic penalties.

Key deadlines

• 31st October – Paper tax return deadline
• 31st January – Online tax return deadline and deadline for paying any tax owed

Penalties for late filing

• £100 fine if you miss the deadline by even a single day.
• Additional £10 daily fines (up to £900) if the return is three months late.
• Further penalties of 5% of tax owed (or £300, whichever is greater) if it’s six months late.
• If you're 12 months late, even harsher fines apply.

Avoid this by setting reminders, gathering documents in advance and filing early.

RIFT tip: Once more, filing via the website gives you a little extra breathing space. Remember to check your account details on the HMRC site regularly. You'll see at a glance what deadlines might be looming.

Tax return penalties

Mistake #5: Getting your personal numbers wrong

This is a broad category, and there are a lot of tiny details that can catch you out. Make sure your National Insurance number and Unique Taxpayer Reference (UTR) number are correct.

RIFT tip: If you lose track of your UTR, you can find it on any correspondence you get from the taxman. Even if you can't remember it, he will.

Mistake #6: Messy Expenses Calculations

Claiming your expenses correctly is a huge part of getting your tax return right. There are strict and complicated rules about what counts as a business expense. Don't cut corners learning them. The taxman has no sense of humour when people claim for things that aren't allowed.

RIFT tip: This mistake can cut both ways. A lot of people miss out on expense claims they could make, and end up paying way too much tax. A quick chat with RIFT could go a long way toward putting cash back in your pocket. Use our free tax calculators for instant estimates.

Mistake #7: Forgetting allowable expenses

If you’re self-employed, not claiming allowable expenses means you’re paying more tax than necessary.

Common business expenses you can deduct

• Office costs (rent, internet, phone bills)
• Travel expenses (fuel, train fares, accommodation for work trips)
• Equipment and tools
• Marketing and advertising costs

Failing to include these means you could be paying too much tax. Keep receipts and detailed records to ensure you claim correctly.

Mistake #8: Not declaring additional income

All taxable income must be declared, not just your main salary. HMRC regularly checks for undeclared earnings, and failing to report additional income can result in penalties.

Types of additional income to declare

• Rental income – If you rent out a property, even just a room, you must declare the income.

Try our rental income tax calculator

• Freelance or side hustle earnings – Selling on platforms like Etsy, eBay, or Fiverr? You may need to pay tax on that income.
  If you’re unsure whether extra income is taxable, check with a tax expert to avoid any costly mistakes.

Mistake #9: Claiming incorrect tax reliefs

Claiming tax reliefs incorrectly is another common tax return mistake that can either result in overpayment or trigger an investigation.

Common tax relief errors

• Personal allowance mistakes – If your income is above £100,000, your personal allowance is reduced. Many taxpayers fail to account for this.
• Pension contributions – Higher rate taxpayers often forget to claim additional tax relief on private pension contributions.
• Charitable donations – If you donate through Gift Aid, you may be able to claim additional tax relief.

Double check what you’re entitled to and ensure all claims are accurate.

Mistake #10: Failing to pay on time

Even if your tax return is filed correctly, you must also pay any tax due on time to avoid penalties.

Late payment penalties

• 5% of unpaid tax if payment is 30 days late
• Additional 5% after 6 months
• Interest charged daily on late tax payments

If you struggle to pay on time, contact HMRC as soon as possible to arrange a payment plan.

Mistake #11: Not telling the whole story

It can take a lot of work to fill in all the blanks when your finances are complicated - but it's worth it. Remember that this information can actually save you money or qualify you for tax relief. Depending on your situation, you might have to give details of things like:

• Life insurance gains.
• Stock dividends.
• Income from property.
• Any benefits you're claiming.
• Income from savings and pensions.

RIFT tip: The self assessment tax return form has a space for "additional information". Remember to use it if you need to add details not covered by the rest of the form.

Mistake #12: Not doing your homework

Filing a self assessment tax return means paperwork. From the money you've got coming in to the miles you're travelling, you need to get used to keeping good business records.

The taxman isn't interested in seeing a rough outline of your finances. He wants the full picture. If you're claiming expenses, you need receipts. If you're billing clients, you need invoices. Depending on the business you're in, there could be a range of details to keep track of. You've got to be able to prove everything you say in your return.

RIFT tip: If you're working PAYE and filing a tax return, you need to keep your records for at least 22 months after the end of the tax year. If you're self-employed, you need to hang onto them for 5 years!

What do I do if I make a mistake on my tax return?

If you realise you've got it wrong, you should always move fast to fix it. In general, you'll have 12 months after the filing deadline to correct any mistakes.

RIFT tip: Our best advice is always to come and talk to us. Even if you've already filed your return, there are ways we can help you correct it. While we're at it, we may be able to save you some money, too.

RIFT are the UK's top tax experts. We can handle everything from one-off self assessment worries to annual tax rebates worth thousands of pounds. The rules of the self assessment game are tricky to learn, but we'll make sure the taxman always plays fair.