First-Time Filer? Here's What You Need to Know About Self Assessment

Completing your first Self Assessment tax return can feel daunting. Between unfamiliar terminology, complex forms, and the fear of making costly mistakes, it's natural to feel overwhelmed. But take a deep breath – thousands of people complete their first tax return every year, and with the right guidance, you can too.

Why Am I Filing for the First Time?

Common reasons for needing to complete your first Self Assessment include:

• You've become self-employed or started freelancing
• Your income has exceeded £100,000
• You've started receiving rental income from a property
• You've become a company director
• You're claiming certain tax reliefs or allowances
• HMRC has written to you asking you to complete a return

If you're unsure whether you need to file, it's better to check with HMRC or an accountant than to risk a penalty for not registering.

Registration: Your First Step

Before you can file your first return, you need to register for Self Assessment with HMRC. You should do this as soon as you know you need to file, but definitely by 5th October following the end of the tax year. For the 2024/25 tax year (which ended on 5th April 2025), that registration deadline has passed, so if you haven't registered yet, do it immediately.

When you register, HMRC will send you:

• A Unique Taxpayer Reference (UTR) number
• An activation code for online filing (this can take up to 10 working days)

You cannot file your return without these, so if you're still waiting for your activation code, contact HMRC urgently.

Understanding the Tax Year

Unlike the calendar year, the UK tax year runs from 6th April to 5th April. Your first tax return will cover the period from 6th April 2024 to 5th April 2025. Any income or expenses outside these dates don't count for this return – they'll be included in next year's.

This can be particularly important if you became self-employed part-way through the year. You only need to declare income from your self-employment that fell within the tax year.

Online or Paper?

Whilst you can technically file a paper return, the deadline for these was 31st October 2025. If you're filing in January, you must file online. The online system is actually quite user-friendly, with built-in calculators and helpful prompts along the way.

The HMRC online system will:

• Calculate your tax bill automatically
• Allow you to save your progress and return later
• Provide instant confirmation when you've submitted successfully
• Show you how much tax you owe (or are owed) immediately

Common First-Timer Mistakes to Avoid

Mistake 1: Mixing Personal and Business Expenses

If you're self-employed, only business expenses are allowable. Your weekly food shop isn't deductible just because you work from home, but the portion of your electricity bill attributable to your home office is.

Mistake 2: Forgetting About Income

You must declare ALL your income, not just your main source. That includes the occasional freelance work, investment income, rental income, or even significant amounts from selling items online.

Mistake 3: Missing the Payment Deadline

Submitting your return by 31st January isn't enough – you also need to pay any tax owed by that date. Different payment methods take different times to reach HMRC, so don't leave this until the last minute.

Mistake 4: Not Keeping Records

HMRC can ask to see evidence for anything on your tax return for up to six years. Keep all invoices, receipts, bank statements, and relevant documents safely stored.

Mistake 5: Guessing Figures

If you're not sure of an exact figure, it's better to take time to find out than to estimate. Inaccurate returns can trigger an HMRC enquiry and potential penalties.

What Records Should I Keep?

Keep records of:

• All income received (invoices, bank statements, payment receipts)
• All allowable business expenses (receipts, invoices, bills)
• Mileage logs if you're claiming vehicle expenses
• Records of any assets bought or sold
• Evidence of pension contributions or charitable donations

How Long Will It Take?

HMRC estimates that a simple tax return takes about two hours to complete. However, if you're self-employed with lots of expenses, or you have multiple income sources, it could take considerably longer.

Don't try to complete your return in one sitting, especially if it's your first time. The online system allows you to save your progress, so you can work through it methodically over several sessions.

Understanding Your Tax Calculation

Once you've entered all your information, the system will calculate:

• Your total income for the year
• Your total allowable deductions
• Your taxable profit
• The tax due on this profit
• Any payments on account for next year

If you owe tax, you'll see three figures:

• The balancing payment (tax owed for 2024/25)
• First payment on account (50% of this year's tax bill, as an advance for next year)
• Second payment on account (the remaining 50%, due by 31st July 2026)

This can be a shock for first-time filers. If you're self-employed and earned £40,000 profit, you might owe around £7,500 in tax – but on 31st January, you'll need to pay £7,500 PLUS another £3,750 (the first payment on account), totalling £11,250.

What If I Can't Afford the Tax Bill?

If you're facing a tax bill you can't pay in full, don't ignore it. HMRC can set up Time to Pay arrangements, allowing you to spread the cost over up to 12 months. However, you must submit your return first and then contact HMRC to arrange this.

Should I Get Professional Help?

Many first-time filers successfully complete their returns independently, particularly if their tax affairs are straightforward. However, consider professional help if:

• You have multiple income sources
• You're self-employed with complex expenses
• You've had capital gains or losses
• You're claiming complex reliefs or allowances
• You simply don't have time to learn the system
• You want peace of mind that everything is correct

The cost of an accountant is far less than the potential cost of mistakes or the stress of trying to navigate the system alone.

Your First-Timer Action Plan

1. Register immediately if you haven't already
2. Gather all documentation (invoices, receipts, bank statements)
3. Set aside dedicated time to work through your return
4. Work methodically through each section of the return
5. Double-check everything before submitting
6. Submit early to avoid last-minute technical issues
7. Arrange payment for any tax owed

You've Got This

Remember, everyone who files Self Assessment was a first-timer once. Take your time, be thorough, and don't hesitate to seek help if you need it. The key is to start early and work through things methodically.

Feeling overwhelmed with your first tax return? MCC Partners specialises in helping new filers navigate the Self Assessment process. Our experienced team at our Gravesend office can guide you through every step, ensuring your return is accurate and submitted on time. Contact us on 01474 619 990 or email This email address is being protected from spambots. You need JavaScript enabled to view it. for expert guidance.