Tax season creeps up fast—and it’s not kind if you’re not ready.

I’ve worked with dozens of people and small businesses over the years, and you'd be shocked how often I hear the same phrase: “I thought I did it right.” But thinking isn’t knowing, and when HMRC starts asking questions, thinking won’t save you.

Let’s walk through the biggest tax return mistakes, so you can avoid them all. Trust me, avoiding just one of these could save you hundreds, or even thousands of pounds.

30-Second Summary

Filing your tax return might sound simple, but many people mess it up without realising. Some guess their numbers. Others miss deadlines or forget to report income. These mistakes can cost you time, money, and peace of mind.

This guide breaks down the most common errors, so you don’t make them. I’ll also share tips I’ve learned from working with clients and accountants in Cambridge and London. If you want to stay out of trouble with HMRC, this one’s for you.

Mistake 1: Guessing Your Numbers Instead of Checking Them Twice

You wouldn’t guess your phone number, right? Then don’t guess your income or expenses either.

Why It’s a Problem

Guesswork leads to incorrect returns. That’s how people underreport income or claim deductions they don’t qualify for. HMRC checks data from banks, employers, and other sources. If your return doesn’t match, you could face penalties.

What You Should Do

• Keep all your receipts and invoices in one place.
• Use accounting software or a spreadsheet to track everything monthly.
• Check your P60, P45, or payslips for accurate figures.

My Experience

I once had a client in East London who guessed his mileage expenses. He was off by 1,200 miles. HMRC didn’t just reject the claim—they opened an audit. That’s how I ended up helping him sort it all out with a tax consultant in East London. Don’t wing it.

Mistake 2: Missing the Deadline (and Paying for It)

You don’t want to be the person rushing through your return on 30th January.