Let’s be honest — filling out a Self Assessment tax return isn’t anyone’s idea of fun. It’s confusing, it’s time-consuming, and there’s this constant worry in the back of your mind: “What if I get it wrong?”
The good news is that most mistakes are completely fixable. The even better news is that most of them are also completely avoidable.
Every year, HMRC sees the same errors over and over again — not because people are trying to cheat the system, but because the whole process is genuinely tricky if you’ve never done it before (and sometimes even if you have).
This guide covers the five most common Self Assessment mistakes UK taxpayers make, why they happen, and exactly what you can do to avoid them. We’ll also show you how to fix things if you’ve already filed and realised you made a mistake.
Let’s jump in.
Mistake 1: Missing the Filing Deadline (or Leaving It Until the Last Minute)
This is the big one. Every year, over a million people in the UK miss the 31 January deadline — and it costs them.
HMRC doesn’t care if you owe tax or not. If your return is even one day late, you’ll get an automatic £100 penalty. If it’s more than three months late, you’ll pay £10 a day (up to a maximum of £900). And it gets worse from there.
Even if you don’t owe HMRC a penny, you still get fined for filing late. It’s not about the money you owe — it’s about the deadline itself.
Why this happens
Most people think, “I’ve got until January, that’s ages away” — and then suddenly it’s mid-January, you haven’t started, and you’re panicking.
Or you start the return and realise you’re missing documents, so you have to wait for your employer to send a P60, or dig through emails to find invoices, or work out what expenses you can actually claim. Before you know it, the deadline has passed.
Some people also assume the deadline is 31 January for filing and paying — which it is — but they don’t realise that if you file on 30 January and something goes wrong, there’s no time to fix it.
How to avoid it
Set yourself an internal deadline of mid-December. That gives you over a month of breathing room if anything goes wrong.
Put a reminder in your phone or calendar for 1 December with the message: “Start your tax return NOW.” Don’t put it off. Even if you can’t finish it in one sitting, getting started early means you’ll know what documents you’re missing and have time to get them.
If you know you’re terrible with deadlines, or your situation is complicated (multiple income streams, rental properties, company dividends), just get an accountant. Seriously. The stress you’ll save is worth the fee, and they’ll often save you more in tax than they charge anyway.
What to do if you’ve already missed it
If you’ve already missed the deadline, file as soon as possible. The penalties increase the longer you leave it, so every day counts.
You can appeal the penalty if you have a reasonable excuse — things like serious illness, bereavement, or a major life event that made it impossible to file on time. “I forgot” or “I was busy” won’t cut it, but genuine circumstances sometimes do.
To appeal, log in to your HMRC account and look for the option to appeal the penalty. You’ll need to explain what happened and provide evidence if you have it (like a doctor’s note).
Mistake 2: Forgetting to Declare All Your Income
This is probably the most common mistake HMRC sees — and it’s one of the most serious, because leaving out income (even accidentally) can look like tax evasion.
People often think, “It’s only a small amount, surely HMRC won’t care?” or “It was cash, so there’s no record of it.” Both of these assumptions are wrong.
HMRC wants to know about all your income, no matter how small or how you received it. If money came into your bank account, your PayPal, or even your pocket, and it wasn’t a gift or a loan, it’s taxable income — and you need to declare it.
Common types of income people forget
Here are the ones we see missed all the time:
Side income from freelancing or gig work — You might have done a few one-off jobs throughout the year, earning £200 here and £500 there. Even if it’s not your main income, it still needs to be declared if the total is over £1,000.
Rental income — If you’re renting out a property, or even just a room in your house, that’s taxable income. The £1,000 property allowance means you don’t pay tax on the first £1,000 of rental income, but you still have to declare it.
Dividends from shares — If you own shares and received dividend payments, these need to be declared. You get a £500 dividend allowance (tax-free), but anything over that is taxable.
Interest from savings accounts — Most people don’t realise that if you earn more than £1,000 in savings interest in a year, you need to declare it on your tax return (unless you’re a basic-rate taxpayer with a £1,000 Personal Savings Allowance).
Tips and commission — If you work in hospitality, sales, or any job where you earn tips or commission that isn’t already taxed through PAYE, you need to include it.
Foreign income — If you earn money abroad — whether it’s rental income from an overseas property, wages from a foreign employer, or interest from a foreign bank account — it’s taxable in the UK if you’re a UK resident.
Cash payments — Just because it was paid in cash doesn’t mean HMRC won’t find out. And even if they don’t, it’s still illegal not to declare it.
Why this is a problem
If HMRC discovers you left out income, they can:
- Charge you the tax you should have paid, plus interest
- Issue a penalty (which can be anywhere from 15% to 100% of the tax owed, depending on whether they think you were careless or deliberate)
- Open a full investigation into your tax affairs
Even if it was a genuine mistake, “I didn’t know I had to declare it” isn’t a defence. It’s your responsibility to know what’s taxable.
How to avoid it
Go through your bank statements for the entire tax year (6 April to 5 April). Look for any payments that aren’t salary, gifts, or refunds. If you received money for doing something — whether it’s selling stuff on eBay, dog walking, tutoring, or renting out your driveway — it’s income.
Make a simple spreadsheet with three columns: Date, Source, and Amount. List everything. Even the £50 you earned from selling old furniture on Facebook Marketplace. It all adds up.
If you’re not sure whether something counts as income, check GOV.UK or ask an accountant. It’s always better to include it and find out you didn’t need to than leave it out and face penalties later.
What to do if you’ve already filed and realised you missed something
Don’t panic. You can amend your tax return within 12 months of the filing deadline.
Log in to your HMRC online account, go to the tax year in question, and select “Amend return.” Add the missing income, and HMRC will recalculate your tax bill. If you owe more, pay it as soon as possible to avoid interest charges.
If it’s been more than 12 months, you’ll need to write to HMRC to correct the mistake. The address is on the GOV.UK website under “Tell HMRC about a mistake in your tax return.”
The key thing is to tell HMRC yourself before they find out. If you come forward voluntarily, they’re usually much more lenient than if they discover it during an investigation.
Mistake 3: Claiming Expenses You’re Not Allowed to Claim
This is the flip side of Mistake 2 — instead of leaving out income, people include expenses they shouldn’t.
It’s tempting to claim everything you can, especially when you see your tax bill and think, “Surely I can reduce this somehow?” But HMRC has very specific rules about what counts as an allowable business expense, and getting it wrong can land you in trouble.
The golden rule: “wholly and exclusively”
For an expense to be allowable, it has to be wholly and exclusively for business purposes. That means:
- If you bought something only for work, you can claim it
- If you bought something for both work and personal use, you can’t claim the full amount (you might be able to claim a proportion)
- If you bought something primarily for personal use, you can’t claim it at all
Common expenses people wrongly claim
Here are the ones that trip people up:
Everyday clothes — Even if you only wear a suit to client meetings, you can’t claim it as a business expense. HMRC’s view is that you could wear it outside of work, so it’s not “wholly and exclusively” for business. The exception is uniforms or protective clothing that you wouldn’t wear in everyday life (like a hi-vis jacket or a nurse’s uniform).
Your daily commute — You can’t claim the cost of travelling from home to your regular place of work. This applies whether you’re self-employed or employed. However, if you travel from your office to visit a client, or from home to a temporary work location, that is claimable.
Lunch and coffee — You can’t claim the cost of your own meals, even if you’re working. HMRC says you’d need to eat whether you were working or not, so it’s not a business expense. The exception is if you’re travelling for work and you’re away overnight — in that case, you can claim reasonable meal costs.
Your home broadband (if you use it personally as well) — If you work from home and use your broadband for both work and personal use (Netflix, social media, etc.), you can’t claim 100% of the cost. You can claim a proportion based on business use, but you need to be realistic. Claiming 90% when you work from home two days a week won’t fly.
Your phone bill (unless it’s a separate work phone) — Same logic as broadband. If you use your mobile for work and personal calls, you can claim a proportion, but not the whole bill.
Capital items you’re still using — If you bought a laptop, a desk, or equipment that you’re still using, you generally can’t deduct the full cost as an expense in the year you bought it. Instead, you claim capital allowances over time. This is a whole separate topic, but the key point is: just because you spent £2,000 on a laptop doesn’t mean you can deduct £2,000 from this year’s profit.
Why this is a problem
If you claim expenses you’re not entitled to, HMRC can:
- Disallow them and recalculate your tax bill (meaning you’ll owe more tax plus interest)
- Charge a penalty if they think you were careless or deliberate
- Open an enquiry into your accounts to check for other errors
How to avoid it
Keep detailed records of everything you claim. That means receipts, invoices, bank statements — anything that proves the expense was for business.
Before you claim something, ask yourself: “Could I use this for personal reasons?” If the answer is yes, think carefully. You might still be able to claim a proportion, but not the full amount.
If you’re not sure, check HMRC’s guidance on allowable expenses (search “Allowable expenses for Self Assessment” on GOV.UK) or ask an accountant.
What to do if you’ve already claimed something you shouldn’t have
If you realise you’ve claimed expenses incorrectly, amend your tax return as soon as possible (within 12 months of the filing deadline).
Remove or adjust the incorrect expenses, and HMRC will recalculate your bill. You’ll likely owe more tax, but by correcting it yourself, you’ll avoid a penalty.
Mistake 4: Entering the Wrong Personal Details (UTR, National Insurance Number, or Business Name)
This sounds like a small thing, but it causes massive problems every year.
Your Unique Taxpayer Reference (UTR) and National Insurance number are how HMRC identifies you. If either of these is wrong — even by one digit — your tax return won’t be accepted. Or worse, it might be accepted but filed under someone else’s record, meaning HMRC thinks you haven’t filed at all.
Common errors with personal details
Mixing up your UTR with someone else’s — If you’re a company director, you’ll have a personal UTR (for Self Assessment) and a company UTR (for Corporation Tax). Make sure you’re using the right one. Your Self Assessment UTR is the 10-digit number of letters from HMRC about your personal tax.
Typing your National Insurance number incorrectly — It’s easy to mix up letters and numbers. Double-check it against your P60, payslip, or any HMRC letter.
Using the wrong business name — If you’re self-employed and you trade under a business name, HMRC’s system can be picky about formatting. Some people use “J Smith Trading” and others use “J. Smith Trading” — and if the system doesn’t recognise it, your return might be rejected.
Not updating your address — If you’ve moved house and haven’t told HMRC, important letters (like your UTR or activation code) might go to your old address. Always update your details as soon as you move.
Why this is a problem
If HMRC can’t match your tax return to your records, they might:
- Reject your submission (meaning you have to resubmit, and if it’s close to the deadline, you might miss it)
- Think you haven’t filed at all (leading to a £100 late-filing penalty)
- Delay processing your tax calculation or refund
How to avoid it
Before you start your tax return, gather these documents:
- Your UTR (on any HMRC letter about Self Assessment)
- Your National Insurance number (on your P60, payslip, or previous tax return)
- Your P60 or P45 from any employment (to make sure your PAYE details are correct)
When you enter these details, double-check every digit. It’s worth spending an extra 30 seconds to make sure it’s right.
What to do if you’ve already submitted with the wrong details
Contact HMRC as soon as you realise. You can call the Self Assessment helpline on 0300 200 3310 and explain the mistake. They’ll advise whether you need to resubmit or whether they can correct it on their end.
If the return was rejected because of incorrect details, you’ll need to correct them and resubmit, but if it’s after the deadline, you may face a penalty (though you can appeal if the mistake was genuine).
Mistake 5: Not Keeping Proper Records (and Paying for It Later)
HMRC doesn’t just trust that the numbers you put on your tax return are correct. They expect you to keep records to prove it — and they can ask to see them at any time.
If you can’t provide proof when HMRC asks for it, they can:
- Disallow your expenses
- Estimate your income (which is usually higher than what you actually earned)
- Charge penalties for careless record-keeping
What records you’re legally required to keep
You must keep records for at least 5 years from the 31 January deadline. For example, if you filed your 2024/25 tax return by 31 January 2026, you need to keep records until at least 31 January 2031.
Here’s what HMRC expects you to keep:
For income:
- Invoices you sent to clients
- Bank statements showing payments received
- Your P60 (if you’re employed as well as self-employed)
- Dividend vouchers (if you receive dividends from shares)
- Rental agreements and rent receipts (if you’re a landlord)
For expenses:
- Receipts and invoices for everything you claim
- Bank or credit card statements showing the payments
- Mileage logs (if you claim vehicle expenses)
- Records of home office use (if you claim working-from-home expenses)
Why this is a problem
If HMRC opens an enquiry into your tax affairs (which they do randomly, or if something looks unusual), they’ll ask for evidence. If you can’t provide it:
- They’ll disallow the expenses and recalculate your tax bill
- You’ll owe the extra tax plus interest
- You might also get a penalty for poor record-keeping
How to avoid it
Set up a simple system at the start of the year. This doesn’t need to be complicated. Here’s what works:
- Use accounting software — Tools like QuickBooks, Xero, or FreeAgent let you scan receipts with your phone, track income and expenses, and generate reports for your tax return. Some are free for very small businesses.
- Keep a dedicated folder — Whether it’s physical or digital, have one place where all your receipts, invoices, and statements go. Label it by tax year.
- Take photos of receipts — Paper receipts fade over time. Take a photo and store it in the cloud (Google Drive, Dropbox, etc.) or email it to yourself. That way, even if you lose the physical receipt, you still have proof.
- Track mileage as you go — If you’re claiming vehicle expenses, keep a mileage log. Every time you drive for work, write down the date, destination, purpose, and miles. There are apps (like MileIQ) that do this automatically using your phone’s GPS.
What to do if you’ve lost your records
If HMRC asks for records and you don’t have them, be honest. Explain what happened and provide whatever evidence you can — bank statements, copies of invoices, etc.
In some cases, HMRC will accept alternative evidence. For example, if you’ve lost receipts but you have bank statements showing the payments, that might be enough.
If you genuinely can’t provide anything, HMRC will make an estimate based on what they think is reasonable — but this is almost always higher than what you actually spent, so it’s worth avoiding this situation if possible.
Bonus Mistake: Not Claiming Tax Relief You’re Entitled To
This isn’t technically a “mistake” in the sense that it’ll get you in trouble, but it’s a mistake in the sense that you’re paying more tax than you need to.
Lots of people don’t realise they can claim tax relief on things like:
- Pension contributions — If you pay into a private pension, you can claim tax relief on the contributions (up to certain limits).
- Charitable donations through Gift Aid — If you donate to charity through Gift Aid, you can claim back the difference between basic rate and higher rate tax.
- Professional subscriptions — If you pay for memberships to professional bodies (like the Law Society, Royal College of Nursing, etc.) that are required for your job, you can claim tax relief.
- Work expenses your employer didn’t reimburse — If you had to buy equipment, uniforms, or tools for work and your employer didn’t pay you back, you might be able to claim tax relief.
These aren’t automatically included in your tax return — you have to actively claim them. If you don’t, you’re essentially giving HMRC free money.
What to Do If You’ve Already Made a Mistake
If you’ve filed your tax return and realised you made a mistake, don’t ignore it. Fixing it now is always better than waiting for HMRC to find it.
Here’s what to do:
If it’s been less than 12 months since the filing deadline
You can amend your tax return online. Just log into your HMRC account, go to the relevant tax year, and select “Amend return.” Make the corrections, and HMRC will recalculate your bill.
If you owe more tax, pay it as soon as possible to avoid interest. If HMRC owes you a refund, they’ll process it and pay you back within a few weeks.
If it’s been more than 12 months
You’ll need to write to HMRC to explain the mistake. The process is called “overpayment relief” if you overpaid tax, or you might need to submit a letter explaining the underpayment if you owe more.
Full details are on GOV.UK — search “Tell HMRC about a mistake in your tax return.”
If you’re worried about penalties
If your mistake was genuine and you correct it as soon as you realise, HMRC is usually lenient. Penalties are typically reserved for careless or deliberate errors, not honest mistakes.
If you do get a penalty, you can appeal it — especially if you have a reasonable excuse (like serious illness or receiving incorrect advice).
Final Thoughts
Self-assessment doesn’t have to be stressful. Yes, there are rules. Yes, it’s easy to make mistakes. But most of those mistakes are avoidable if you:
- Start early — Don’t wait until January
- Keep good records — Receipts, invoices, bank statements
- Double-check everything — Your personal details, your income, your expenses
- Ask for help if you need it — Whether that’s an accountant, HMRC’s helpline, or online resources
And if you do make a mistake, fix it quickly. HMRC would rather you correct things yourself than discover them later.
If all of this still feels like too much — or if you just don’t want to spend hours wrestling with forms and deadlines — that’s what we’re here for.
At Applegrow Financial Advisors, we handle Self Assessment tax returns every single day. We’ll make sure everything is filed correctly, on time, and that you’re not paying a penny more than you need to.
Get in touch and let us take it off your hands.
Need Help With Your Self Assessment?
Whether you’ve made a mistake, missed a deadline, or just want someone else to handle the whole thing, we’re here to help.
We specialise in Self Assessment for self-employed individuals, landlords, and company directors — and we’ll make sure your tax return is accurate, compliant, and stress-free.
Contact us today to find out how we can help.
