Self Assessment Tax Return: Guide for First-Timers

Filing your first Self Assessment tax return can feel overwhelming. There are forms to fill, deadlines to meet, and the nagging worry that you might get something wrong. But here is the good news: once you understand the process, it is far more straightforward than most people expect. I am Penny, your AI bookkeeper at Accounted, and I have helped thousands of first-timers navigate their way through Self Assessment without breaking a sweat.

In this guide, I will walk you through absolutely everything you need to know — from working out whether you even need to file, right through to submitting your return and paying your tax bill. Let us get started.

Do You Actually Need to File a Self Assessment Return?

Not everyone in the UK needs to file a Self Assessment tax return. If you are employed and your tax is handled entirely through PAYE, you probably do not need to worry about it. However, there are quite a few situations where HMRC requires you to file one.

You will need to submit a Self Assessment return if you:

• Are self-employed as a sole trader and earned more than £1,000 in a tax year
• Are a partner in a business partnership
• Have untaxed income, such as rental income, tips, or commission
• Earned more than £150,000 in a tax year (even if employed)
• Need to pay the High Income Child Benefit Charge
• Have income from abroad that you need to pay UK tax on
• Are a company director (unless it is a non-profit organisation with no pay)
• Receive income from savings, investments, or dividends above certain thresholds
• Need to claim certain tax reliefs

If any of these apply to you, the first thing you need to do is register with HMRC. You can find out more about the registration process in our guide on how to register for Self Assessment online.

HMRC provides a helpful tool on their website to check whether you need to file. You can access this at GOV.UK — Check if you need to send a Self Assessment tax return.

Registering for Self Assessment

Before you can file a return, you need to register with HMRC. The process differs depending on your circumstances.

If you are newly self-employed, you need to register for Self Assessment and Class 2 National Insurance by 5 October following the end of the tax year in which you started your business. For example, if you started freelancing in June 2025, you would need to register by 5 October 2025.

You can register online through the HMRC Self Assessment registration page. You will need your National Insurance number and some basic personal details.

Once you have registered, HMRC will send you a Unique Taxpayer Reference (UTR) number by post. This usually takes about 10 working days, though it can sometimes take longer. You will need this number to file your return, so do not leave registration to the last minute.

If you need to file for another reason (such as rental income or the High Income Child Benefit Charge), you register using form SA1, which is also available on the GOV.UK website.

After receiving your UTR, you will also need to set up a Government Gateway account if you plan to file online. This involves another verification step where HMRC sends you an activation code, which can take another 10 working days to arrive.

The key takeaway here is this: register early. Do not wait until January to sort this out. You can learn more about the full timeline in our Self Assessment deadlines guide.

Gathering Your Records and Information

Before you sit down to complete your return, you need to have all your financial information to hand. The better organised you are, the faster and less stressful the process will be.

Here is what you will typically need:

Income records:

• Your total income from self-employment (sales invoices, bank statements)
• P60 or P45 from any employment
• P11D if you received any benefits in kind from an employer
• Records of rental income
• Bank and building society interest statements
• Dividend vouchers from share investments
• Any other income (such as pension income, foreign income, or casual earnings)

Expense records (if self-employed):

• Receipts and invoices for business expenses
• Mileage logs if you claim vehicle expenses
• Records of use-of-home costs if you work from home
• Details of any capital purchases (equipment, vehicles, etc.)

Other information:

• Your UTR number
• Your National Insurance number
• Details of any pension contributions you made
• Gift Aid donation records
• Student loan details (if applicable)
• Details of any capital gains from selling assets

If you have been using Accounted throughout the year, all of your income and expense records will already be neatly organised and ready to go. Check out our features page to see how we make record-keeping effortless.

Completing Your Tax Return Step by Step

The main Self Assessment form is the SA100. Depending on your circumstances, you may also need to complete supplementary pages. The most common ones are:

• SA103S or SA103F — Self-employment (short or full version)
• SA105 — UK property income
• SA106 — Foreign income
• SA108 — Capital gains
• SA110 — Tax reliefs

If you are filing online (which I strongly recommend), the system will guide you through which sections you need to complete based on your answers to the initial questions.

Here is the general flow:

Step 1: Personal details — Confirm your name, address, UTR, and National Insurance number.

Step 2: Tailor your return — Answer a series of yes/no questions about your income sources. This determines which sections of the return you need to fill in.

Step 3: Report your income — Enter details of all your income. If you are self-employed, this is where you report your turnover and expenses. For a detailed breakdown of how your tax is calculated, read our guide on how to calculate your tax bill as a sole trader.

Step 4: Tax reliefs and allowances — Enter details of pension contributions, Gift Aid donations, and any other reliefs you are claiming.

Step 5: Review and submit — Check everything carefully, then submit your return.

The online system will automatically calculate your tax bill based on the information you provide.

Understanding Your Tax Bill

Once your return is submitted, HMRC will tell you how much tax you owe. Your bill is made up of several components:

Income Tax is calculated on your total taxable income (after deducting your Personal Allowance of £12,570). The rates for 2025/26 are:

• Basic rate: 20% on income between £12,571 and £50,270
• Higher rate: 40% on income between £50,271 and £125,140
• Additional rate: 45% on income over £125,140

Class 2 National Insurance is a flat weekly amount for self-employed people (£3.45 per week for 2025/26, though this may not be due if your profits are below the Small Profits Threshold).

Class 4 National Insurance is charged on self-employed profits:

• 6% on profits between £12,570 and £50,270
• 2% on profits above £50,270

Student loan repayments may also be collected through Self Assessment if applicable.

One thing that catches many first-timers off guard is payments on account. If your tax bill is more than £1,000 (and less than 80% of it was collected at source), HMRC will ask you to make advance payments towards next year's bill. This effectively means you pay one and a half times your normal bill in the first year. Our detailed guide on payments on account explains exactly how this works and what you can do about it.

Key Deadlines You Cannot Afford to Miss

Missing a deadline can result in automatic penalties, so it is crucial to know the important dates:

• 5 October — Deadline to register for Self Assessment if you are newly self-employed
• 31 October — Deadline for paper tax returns (though hardly anyone files on paper these days)
• 31 January — Deadline for online tax returns AND payment of your tax bill
• 31 January — First payment on account due (if applicable)
• 31 July — Second payment on account due (if applicable)

If you miss the 31 January filing deadline, you will receive an automatic £100 penalty — even if you do not owe any tax. Further penalties accumulate the longer your return is overdue. For a complete breakdown of what happens when you miss deadlines, have a look at our Self Assessment penalties guide.

Common Mistakes First-Timers Make

Having helped countless people through their first Self Assessment, I can tell you the most common pitfalls to avoid:

Not keeping records throughout the year. Trying to reconstruct a year's worth of finances in January is stressful, time-consuming, and prone to errors. Use an app like Accounted to track your income and expenses as you go. You can sign up here and start keeping organised records from day one.

Forgetting to claim allowable expenses. Many first-timers pay more tax than they need to because they do not claim all their legitimate business expenses. Common overlooked expenses include use-of-home costs, professional subscriptions, and phone bills. Check out our guide on tax deductions for sole traders to make sure you are not leaving money on the table.

Mixing personal and business finances. This makes it much harder to work out your business income and expenses accurately. Open a separate business bank account from the start.

Not saving money for tax. A good rule of thumb is to set aside 25-30% of your income for tax. Do this every time you receive a payment, not just when the bill arrives.

Filing inaccurate figures. Double-check everything before you submit. HMRC can charge penalties for inaccurate returns, and if your figures do not match the information HMRC already holds (such as bank interest data they receive from banks), it could trigger an enquiry.

What Happens After You File

Once your return is submitted, HMRC will confirm receipt and show you your tax calculation. You then need to pay any tax owed by 31 January (for returns relating to the previous tax year).

You can pay your Self Assessment bill in several ways:

• Online banking or bank transfer
• Direct Debit
• Debit card online
• At your bank or building society
• Through your PAYE tax code (if the amount is under £3,000 and you file by 30 December)

If you cannot afford to pay the full amount, do not panic. HMRC offers Time to Pay arrangements where you can spread your payments over up to 12 months. You can set this up online if your bill is under £30,000.

Getting Help When You Need It

Filing your first Self Assessment does not have to be a solo endeavour. There are plenty of resources available:

• HMRC helpline — You can call HMRC on 0300 200 3310 for general Self Assessment queries
• HMRC webchat — Available through your Government Gateway account
• Accounted — Our pricing plans include AI-powered support that can guide you through the entire process

The most important thing is not to bury your head in the sand. If you are unsure about something, ask. It is always better to file on time with your best estimate than to miss the deadline because you were waiting for perfection.

Final Thoughts

Your first Self Assessment tax return is one of those things that seems far more intimidating than it actually is. Yes, there is paperwork. Yes, there are deadlines. But the process is logical, and once you have done it once, it becomes routine.

Start by registering with HMRC as early as possible. Keep your records organised throughout the year. Understand what expenses you can claim. And file your return well before the January deadline — you will thank yourself for not leaving it to the last minute.

If you would like a helping hand, I am always here. Accounted is designed to make Self Assessment as painless as possible, handling the bookkeeping and number-crunching so you can focus on running your business. Here is to your first filing being smooth, stress-free, and maybe even a little satisfying.

Accounted files your Self Assessment directly to HMRC, with your return pre-populated from your records. See Self Assessment filing →