Self-Assessment tax return deadline – last minute tips and common mistakes to avoid

The Self-Assessment deadline creates pressure every year. You sit down with good intentions, then realise how much detail the return needs. Figures blur together. Paperwork hides in folders. Time slips away.

This guide walks you through what matters most when the deadline looms. You’ll see where people trip up, how to avoid costly errors, and when expert help makes sense.

Why the deadline matters

The online Self-Assessment deadline falls on 31 January. Miss it and HMRC charges an automatic late filing penalty. That applies even when no tax is due.

Leave tax unpaid and interest starts building straight away. Additional penalties follow the longer things drag on. None of these charges help your business or personal finances.

Ask yourself this – would you willingly hand over money for avoidable mistakes?

What you still need to file on time

Last-minute filing works best when you focus on essentials. Getting these right puts you back in control.

You’ll need:

·       Your Unique Taxpayer Reference – without it, you won’t submit

·       Your National Insurance number

·       Income details from every source – employment, self-employed work, rental income, dividends, interest

·       Allowable expenses and reliefs

·       Records of tax already paid, such as PAYE or CIS deductions

Missing details slow everything down. They also increase the chance of errors that surface later. A calm checklist now beats frantic fixes later.

Common mistakes that cost you money

Pressure leads to rushed decisions. These mistakes crop up again and again and often trigger penalties or enquiries.

Guessing figures

When time feels tight, estimates seem like a shortcut. HMRC expects accurate figures backed by records. Guessing increases the chance of follow-up questions long after submission.

Would you rather spend ten more minutes now or weeks dealing with HMRC later?

Forgetting income streams

Side income often slips through – freelance work, rental income, savings interest, or dividends. HMRC receives data from banks, employers, and other sources and compares it against your return.

Leaving income out rarely stays hidden and sorting it later takes more effort.

Claiming the wrong expenses

Expenses must relate wholly and exclusively to your business. Personal costs claimed as business ones attract attention.

Ask yourself a direct question – would you feel comfortable explaining this claim to HMRC?

Missing payments on account

Payments on account catch many people out. They cover part of next year’s bill and fall due alongside your balancing payment.

Seeing a larger bill than expected causes stress. Knowing what’s coming helps you plan cash flow with confidence.

Practical last-minute tips that help

Even close to the deadline, smart steps reduce risk and stress.

Slow down and check

Speed feels helpful under pressure, yet accuracy matters more. Take time to review each section carefully. A single error often creates bigger problems later.

Use last year’s return as a sense check

Last year’s figures provide a useful benchmark. Large changes deserve clear explanations and solid records. That review helps spot missing income or duplicated expenses.

Keep notes for next year

Write down missing paperwork, unclear figures, and lessons learned. That simple habit turns this year’s stress into next year’s smoother process.

What happens after you submit?

Once you submit, HMRC sends confirmation. Save it. That receipt proves submission and gives peace of mind.

Tax due remains payable by 31 January. Late payment attracts interest straight away. Early action on payment plans reduces pressure and limits charges.

A quick review after submission also helps you plan better for the year ahead.

When professional help makes sense

Self-Assessment involves more than filling boxes. It’s about paying the right tax and understanding your position.

Professional support makes sense when:

·       Your income comes from several sources

·       You feel unsure about expenses or reliefs

·       You’ve faced penalties before

·       You want clarity, not guesswork

Good advice removes doubt and often saves money as well as time.

Talk to Liondaris & CO

Still wrestling with your Self-Assessment? Unsure about figures, deadlines, or payments?

Liondaris helps individuals and business owners handle Self-Assessment with clarity and confidence. You’ll get straight answers, practical advice, and clear next steps.

Ask yourself one final question – do you want another late night worrying about tax, or do you want it handled properly?

Get in touch with our self-assessment experts today and take the pressure off.

Contact us - we’re ready to help!