Self-Assessment HMRC — how to file taxes in the UK

Introduction / Who is it for

Self-Assessment is the annual tax return in the UK. Most employees on PAYE (Pay As You Earn) do NOT have to file it — their employer deducts taxes for them. Self-Assessment applies to:

• Self-employed individuals (self-employed, sole trader)
• Partners in partnerships
• Individuals with income over £100,000 per year
• Those receiving rental income from property (>£1,000/year)
• Income from stocks, crypto, dividends above allowance
• Individuals with foreign income (e.g., from Poland)
• High-income spouses receiving Child Benefit

UK Tax Year

The UK tax year: April 6 of year N to April 5 of year N+1. (Unlike in Poland, where the year = calendar year.)

Deadlines

• October 5 — deadline for registering for Self-Assessment (if it's your first time)
• October 31 — deadline for paper filing (rarely used)
• January 31 — deadline for online filing + tax payment
• July 31 — second payment on account (advance for the next year)

Missing the deadline: a penalty of £100 automatically, plus interest, plus additional penalties after 3 months.

Step by Step

Step 1: Register for Self-Assessment

1. Go to gov.uk/register-for-self-assessment
2. Fill out the form (NIN required)
3. You will receive a UTR (Unique Taxpayer Reference) — a 10-digit number, by post in 2-4 weeks
4. You will receive a Government Gateway ID for online login

Step 2: Gather Documents

• P60 / P45 / P11D (from employer)
• Self-employment documentation (invoices, business bank statements, expense records)
• Bank statements for the tax year
• Documents for foreign income (from Poland: PIT-37 / PIT-36)
• Business expense receipts (fuel, phone, equipment, office rent)

Step 3: Submit Online Return

1. Log in at gov.uk/log-in-file-self-assessment-tax-return
2. Complete the sections:
   • Employment income
   • Self-employment income
   • Foreign income
   • Rental income (UK property)
   • Dividends and interest
3. Enter deductions / allowances (Personal Allowance, Marriage Allowance, etc.)
4. The system will calculate the tax due
5. Pay by card or bank transfer by January 31

Personal Allowance

In 2026, everyone has £12,570 of income per year tax-free. Tax rates above this threshold:

• 20% — £12,571 - £50,270
• 40% — £50,271 - £125,140
• 45% — above £125,140

Plus National Insurance (12% for employees; Class 2 and 4 for the self-employed).

Self-employed: Deductible Costs

• Business travel tickets (NOT commuting to a permanent office)
• Fuel (45p/mile for up to 10,000 miles, 25p/mile above)
• Part of rent/utilities if working from home (use of home as office)
• Business phone, internet
• Office equipment
• Industry subscriptions
• Professional indemnity insurance
• Advertising and marketing

Polish Income (foreign income)

If you are a UK tax resident (>183 days in the year), you MUST report income from all over the world, including from Poland. Thanks to the Polish-British double taxation agreement (1976):

• Income taxed in Poland — usually credited against UK tax
• You must include a Foreign Tax Credit (FTC)
• Polish PIT-37 as proof of tax paid

Costs

Filing Self-Assessment is FREE. You only pay the tax due.

Accountant: £200-800 per year. Worth it for complicated cases (foreign income, multiple businesses).

Common Mistakes

• Not registering by October 5 — £100 penalty
• Filing after January 31 — automatic £100 penalty + interest
• Not reporting Polish income (HMRC has information exchange agreements with Poland)
• Mixing personal and business expenses
• Lack of expense records for 6 years (HMRC requirement)
• Not paying Payment on Account in July

What if HMRC orders an audit

• Keep all documents for 6 years
• HMRC has 12 months to start an audit from the date of filing (longer in case of suspected fraud)
• You can hire an accountant for the audit (qualified: ACA, ACCA, CTA)
• Be cooperative — penalties for non-cooperation are higher than for genuine mistakes