If you’ve ever stared down a Self Assessment tax return (SATR) and thought “this feels harder than it needs to be” — you’re not alone. Every year, thousands of business owners and landlords run into trouble with their tax return, often through simple mistakes that HMRC doesn’t take lightly.
The good news? Most of these errors are completely avoidable — especially with a bit of prep and the right tools (hello Xero!). So, let’s walk through the most common SATR slip-ups and how you can steer clear of them.
1. Missing the Deadline
Let’s start with the obvious one: 31 January is the big deadline. Miss it, and HMRC slaps on an automatic £100 fine — even if you don’t owe any tax.
How to avoid it:
Set reminders in your calendar (we recommend one for the start of January, and another mid-Jan for a buffer).
Use Xero to keep track of income and expenses all year round — so you’re not scrambling for info last minute.
File early — you can submit your Self Assessment anytime from 6 April. That gives you loads of breathing room, and if you haven’t been putting tax aside as you go, early filing means more time to save up before the 31 Jan payment deadline. You still don’t need to pay until then, even if you file in the spring or summer.
2. Using the Wrong Tax Code or UTR
Your Unique Taxpayer Reference (UTR) and National Insurance number are vital. Mistyping them or entering a code from an old job or PAYE scheme can hold up your whole return.
Avoid it by:
Double-checking any codes against HMRC letters or your Xero dashboard (if you’ve linked it to your HMRC account).
Not guessing — if in doubt, contact HMRC to confirm your UTR or NI number.
3. Not Declaring All Income
This one trips up a lot of people. It’s easy to forget about a bit of side income, bank interest, dividends, or rental income — but HMRC won’t.
Here’s what counts as income:
Money from your business or freelance work
PAYE income (if you had a part-time job too)
Rental income
Dividends or savings interest
Grants (like SEISS)
Foreign income
Top tip: Use Xero’s bank feeds to pull in all your transactions automatically — that way, nothing slips through the cracks.
4. Claiming the Wrong Expenses
We’ve all heard the classic “can I claim my dog?” myth (no, you can’t — unless it’s a trained security dog). But lots of folks genuinely struggle to figure out what counts as an allowable expense.
Common trip-ups:
Claiming full costs for items that are partly personal (like mobile phones)
Forgetting to claim for use of home as office
Not using the right method for vehicle costs
Xero can help:
Tag and categorise expenses as you go, then use reports to see what you’re spending and what you can claim. And if you’re not sure? That’s a perfect moment to check with your accountant.
5. Ignoring Payments on Account
If your last tax bill was over £1,000, HMRC may ask you to make payments on account — two advance payments towards next year’s bill.
Miss this, and you could be hit with late payment interest or a bigger tax bill than expected.
Solution:
Check your HMRC statement or ask your accountant if you’re due to make payments on account.
Xero’s reports can help forecast what you might owe, so you’re not caught short.
6. Not Keeping Proper Records
HMRC requires you to keep records for at least 5 years after the 31 January deadline. That includes invoices, receipts, bank statements, and mileage logs.
Relying on spreadsheets or paper files? That’s risky. One spilt coffee or lost laptop, and you’re in hot water.
A smarter way:
Store everything in the cloud with Xero — photos of receipts, digital invoices, the lot.
Use apps like Hubdoc, Dext, or Apron to snap receipts and invoices on the go and send them straight to Xero — no more digging through shoeboxes or inboxes.
Set up bank feeds and invoice tracking to automate record-keeping and make tax time much less stressful.
7. Doing It All Alone (and Stressing About It)
Sure, some people are confident handling their tax returns solo. But if it’s causing you stress, costing you hours, or leaving you guessing, it’s worth getting support.
Consider:
Using Xero to keep things tidy and reduce errors
Working with a qualified accountant (like us!) to double-check everything
Asking for help early, especially if your business has changed
Don’t Let SATR Stress You Out
A Self Assessment tax return doesn’t have to be a drama. Most of the common errors come from rushing, guessing, or using old-school systems that don’t give you the full picture.
With a bit of prep, the right software, and support when you need it, your SATR can be surprisingly stress-free.
Need help sorting your records, setting up Xero, or just want us to handle the whole thing? Book in a chat — we’re here to make tax time easy.
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