Self Assessment Tax Return Guide 2026: Everything UK Sole Traders Need to Know

Self Assessment can feel overwhelming, but it doesn't have to be. This guide breaks down everything you need to know to file your tax return with confidence.

What is Self Assessment?

Self Assessment is how HMRC collects Income Tax from people who don't have tax deducted automatically through PAYE.

If you're a sole trader, freelancer, or have untaxed income, you'll file a Self Assessment return each year telling HMRC what you earned and what you owe.

The good news? Once you understand the system, it's straightforward. Most sole traders can complete their return in under an hour with the right preparation.

Key Deadlines for 2025/26

Mark these dates in your calendar:

Miss the 31 January deadline and you'll face an automatic £100 penalty — even if you owe nothing.

Do You Need to File?

You must file a Self Assessment return if you:

• Are self-employed and earned over £1,000
• Are a partner in a business partnership
• Have untaxed income (rental income, foreign income, savings interest)
• Earned over £100,000 in the tax year
• Need to pay the High Income Child Benefit Charge

Not sure? Use HMRC's online tool to check.

Understanding Your Tax Bill

Your tax is calculated on taxable profit:

Total Income − Allowable Expenses − Personal Allowance = Taxable Income

2025/26 Tax Rates

National Insurance

Self-employed people pay two types of NI:

Class 2 NICs — £3.45 per week if profits exceed £12,570

Class 4 NICs — 9% on profits between £12,570 and £50,270, then 2% above

These are calculated automatically when you submit your return.

Payments on Account

Here's something that catches many people out.

If your tax bill exceeds £1,000, HMRC will ask for advance payments towards next year's bill:

• 31 January — 50% of last year's bill
• 31 July — Another 50%

This means in your first year of high earnings, you might pay 150% of your tax bill. Plan ahead.

How to File Your Return

Step 1: Register

Get your Unique Taxpayer Reference (UTR) from HMRC. This can take a few weeks, so don't leave it late.

Step 2: Gather Records

Collect all your income and expense records for the tax year.

Step 3: Complete Your Return

Use HMRC's online portal or accounting software like dynamik.app.

Step 4: Submit

File online by 31 January.

Step 5: Pay

Pay what you owe by the same date.

Common Mistakes to Avoid

❌ Missing the deadline — Automatic £100 penalty

❌ Forgetting allowable expenses — You're paying more tax than necessary

❌ Poor record keeping — HMRC can investigate up to 6 years back

❌ Mixing personal and business expenses — Keep them separate

❌ Forgetting Payments on Account — Budget for next year's advance payments

Make Tax Time Easier

Good accounting software tracks your income and expenses throughout the year, so when January comes, you're ready.

With dynamik.app, you can:

• Track income and expenses automatically
• Generate reports for your accountant
• Stay MTD compliant
• Calculate your tax estimate in real-time

Start your free trial →