Top 10 Tips on Filing Your Self Assessment Tax Return

As the Self Assessment deadline approaches, millions of taxpayers rush to file at the last minute. While being deadline-driven isn’t unusual, leaving things late increases the risk of mistakes and, more importantly, missing opportunities to reduce your tax bill.

If you need to complete a Self Assessment tax return, these ten practical checks will help you avoid common traps, stay compliant with HMRC, and ensure you’re not paying more tax than necessary.

1. Make Sure You Can Access the System Before You Start

Before reviewing figures or allowances, confirm that you can actually submit your return.

You’ll need:

• Your Unique Taxpayer Reference (UTR)
• Access to your Government Gateway account

If you’re not registered, or you’ve lost your details, recovery codes can take time to arrive. System access issues are one of the most common reasons people miss the deadline entirely so deal with this first.

2. Check Capital Gains Tax Calculations Carefully

If you sold assets such as shares, property, or crypto, don’t assume the Self Assessment system has calculated everything correctly.

In particular:

• Capital gains made after 30 October 2024 may not be calculated accurately
• You may need to manually adjust figures using HMRC’s CGT calculator

Errors here can lead to underpaying tax and follow-up questions later — or overpaying without realising it.

3. Understand How Frozen Tax Thresholds Affect You

Income tax thresholds have been frozen, meaning pay rises can quietly push you into higher or additional-rate tax.

This can trigger:

• A requirement to file a tax return for the first time
• Loss of the personal savings allowance
• The need to reclaim higher-rate pension or gift aid relief

Many people overpay tax simply because they don’t realise a threshold change has altered their position.

4. Review Child Benefit and the High Income Charge

If either you or your partner earns over £60,000, the High Income Child Benefit Charge (HICBC) may apply.

Self Assessment allows you to:

• Calculate what needs to be repaid
• Reclaim Child Benefit if income later falls
• Opt to pay the charge through PAYE in some cases

Ignoring this is a common reason HMRC contacts taxpayers unexpectedly.

5. Declare Crypto Gains — Even If They’re Small

Crypto assets are firmly on HMRC’s radar.

You need to consider:

• Capital gains on disposals
• Whether total gains exceed your annual CGT allowance
• Reporting losses so they can be offset against future gains

Even if no tax is due, reporting losses now can reduce future tax bills.

6. Don’t Overlook Income From Online Selling

If you sell goods for profit on platforms like eBay, Etsy, or Vinted, this may count as trading income — not casual selling.

Key points:

• A £1,000 trading allowance applies
• Profits above this must be declared
• Platforms now share data with HMRC, but you remain responsible

This is an increasingly common area for unexpected tax bills.

7. Take Extra Care With Pension Contributions

Pensions are one of the most valuable tax-saving tools — but also one of the easiest places to make mistakes.

Higher-rate taxpayers should:

• Ensure higher-rate relief is claimed where not automatic
• Enter gross contributions, including tax relief

Getting this wrong can mean missing hundreds or thousands in relief.

8. Check Whether You Owe Payments on Account

If your tax bill exceeds £1,000, you may be required to make payments on account toward next year’s bill.

This often:

• Doubles the amount due in January
• Comes as a shock to first-time filers

If your income has dropped, you may be able to reduce these — but only if you act.

9. Plan How You’ll Pay — Even If You Can’t Pay in Full

Submitting your return doesn’t require immediate payment.

If you can’t afford the bill:

• You may be eligible for a Time to Pay arrangement
• This spreads payments over several months
• Interest applies, but avoids penalties and enforcement action

Missing payment without agreement is far more costly.

10. Use Your Tax Return as a Financial Planning Tool

Once you’ve filed, don’t stop there.

Your tax return often highlights:

• Overexposed savings or investments
• Missed allowances
• Complex income streams that could be simplified

Sheltering income in ISAs or pensions can reduce future tax bills and even remove the need to file at all.

Final Thought

A Self Assessment tax return isn’t just an administrative obligation it’s a snapshot of your financial efficiency.

Filing accurately, understanding thresholds, and actively claiming reliefs can make a meaningful difference to how much tax you pay not just this year, but in the years ahead.

If you’re a UK landlord filing Self Assessment, speak to us before you submit. We’ll review your return and make sure you’re not overpaying tax, contact us today.

Thandi Nicholls Ltd
Creative Industries Centre
Glaisher Drive
Wolverhampton
West Midlands
WV10 9TG