Filing a Self-Assessment Tax Return in the UK: A Complete Guide for First-Timers

Filing a Self-Assessment Tax Return in the UK: A Complete Guide for First-Timers

Filing your first self-assessment tax return in the UK can feel overwhelming. From understanding HMRC requirements to knowing which expenses you can claim, there's a lot to take in. But don't worry—this guide will walk you through everything you need to know as a first-time filer in 2025.

By the end, you’ll have a clear, step-by-step process and the confidence to handle your UK tax return like a pro

What Is a Self-Assessment Tax Return?

Self-assessment is the system used by HM Revenue and Customs (HMRC) to collect income tax. Instead of your tax being automatically deducted (as in PAYE), you declare your income and expenses yourself and pay any tax due.

You must submit a tax return if you:

  1. Are you self-employed or a sole trader
  2. Earned more than £100,000
  3. Receive income (e.g., from renting out property, freelance work, or dividends)
  4. Are a partner in a business partnership
  5. Need to pay capital gains tax

For more eligibility details, see HMRC'S full list .

How to Register for Self-Assessment (First-Time Only)

Before filing your first return, you need to register with HMRC. Here's how:

  1. Create a Government Gateway account: You can register online at HMRC's website
  2. Receive your Unique Taxpayer Reference (UTR): This will be sent by post within 10 days.
  3. Activate your account: Once you receive your UTR and activation code, you can log in and begin your return.

Deadline to register: 5 October 2025 (if you're filing for the 2024/25 tax year).

Important Tax Deadlines

  1. 31 October 2025: Paper return deadline
  2. 31 January 2026: Online return and payment deadline

Late filing results in a minimum £100 fine, even if you owe no tax.

Documents You'll Need to File

Gather these before you begin:

  1. Unique Taxpayer Reference (UTR)
  2. National Insurance number
  3. Details of income (employment, self-employment, rental, investments)
  4. Records of allowable expenses
  5. P60, P45, or pension statements
  6. Bank interest statements (if applicable)
  7. Student loan repayment details

Filing Your First Self-Assessment Tax Return Online

Step 1: Log In to HMRC Online

Use your Government Gateway ID to log in at HMRC Online Services.

Step 2: Start Your Tax Return

Click on 'Complete your tax return' and answer questions about your income, expenses, and tax reliefs.

Step 3: Enter Your Income

You'll need to report:

  1. Employment income
  2. Self-employment or partnership income
  3. Rental income
  4. Investment/dividend income
  5. Any foreign income

Step 4: Claim Your Expenses

You can deduct allowable business expenses such as:

  1. Office supplies
  2. Travel and mileage
  3. Marketing and advertising
  4. Home office expenses

Be honest and keep records for at least five years in case HMRC audits your return.

Step 5: Add Tax Reliefs and Deductions

Include any:

  1. Charitable donations
  2. Pension contributions
  3. Marriage allowance transfer

Step 6: Submit and Pay

Once complete, HMRC calculates your tax bill. Submit your return and pay the balance due by 31 January 2026.

Payment options:

  1. Bank transfer
  2. Debit/corporate credit card
  3. Direct debit

Common First-Time Filing Mistakes to Avoid

  1. Missing the deadline: Set reminders to avoid penalties.
  2. Using incorrect figures: Always double-check your numbers.
  3. Overclaiming expenses: Only claim what’s genuinely used for business.
  4. Not keeping records: Keep receipts, invoices, and logs for five years.

Why Choose Selftax?

First-time filers love Selftax because it makes everything easier:

  1. Easy-to-follow interface tailored for UK self-assessment
  2. Built-in checks to avoid mistakes
  3. Affordable pricing with no hidden costs
  4. Secure document uploads
  5. Real-time customer support

Whether you're self-employed, renting out property, or just new to the system, Selftax ensures you stay compliant without the headache.

Final Thoughts

Filing your first self-assessment tax return may feel like a big task, but it's completely manageable with the right guidance. By starting early, staying organised, and using a simple platform like Selftax.co.uk, you can take full control of your tax responsibilities.

Don't wait until the last minute. Register now, gather your documents, and make your first tax return stress-free.