A Complete Guide to Your Self Assessment Form
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Filing your first Self Assessment form can feel intimidating. Once you understand what the form covers and what you need to have ready, the process becomes far more manageable.
This guide walks you through what a Self Assessment form is, who needs to submit one, the key dates you need to know, and how to work through the form step by step.
What Is a Self Assessment Form?
A Self Assessment form is the document you use to report income to HMRC that has not already been taxed automatically. Most employees pay tax through PAYE, so their employer deducts it before they receive their pay. Self Assessment exists for income that falls outside that system, such as self-employment earnings, rental income, or investment gains.
HMRC uses the information you submit to work out how much tax you owe. In some cases, the calculation shows that you are owed a refund instead.
Do You Need to Submit a Self Assessment Form?
You will generally need to file a Self Assessment tax return if any of the following apply to you:
- You are self-employed or work as a sole trader and your income exceeds the £1,000 trading allowance
- You earn rental income as a landlord
- You bring in more than £1,000 from freelance work or a side business alongside your main job
- You are a partner in a business partnership
- You receive income from investments, dividends, or savings above the relevant thresholds
If you are unsure whether you need to file, HMRC provides an online checking tool that can confirm your position. If you filed a Self Assessment return in a previous year, it is also worth checking whether the Making Tax Digital (MTD) rules for Income Tax apply to you, since they change how you record and report income going forward.
Key Self Assessment Deadlines
Your deadline depends on how you plan to register, file, and pay. The table below outlines the main dates for the 2025/26 tax year.
Action | What It Means | Deadline |
|---|---|---|
Register for Self Assessment | Required if you need to file for 2025/26 but did not file the previous year | 5 October 2026 |
Submit a paper tax return | Applies if you choose to file using a paper form | 31 October 2026 |
Submit online to pay through your tax code | Lets HMRC collect tax owed via PAYE if your bill is under the qualifying threshold | 30 December 2026 |
Submit a tax return online | The standard deadline for most filers | 31 January 2027 |
Pay your tax bill | Payment is due even if you filed earlier | 31 January 2027 |
Missing any of these dates can trigger an automatic penalty, even if you do not owe any tax. It’s worth filing well ahead of the deadline rather than leaving it until the last week of January.
If your tax bill comes to more than £1,000, you may also need to make payments on account toward your next year’s bill. These advance payments are typically due on 31 January and 31 July.
How to Fill Out a Self Assessment Form
Step 1: Gather Your Documents First
Before you start, collect everything you’ll need. Trying to find missing paperwork halfway through the form only adds stress. Have the following ready:
- Your Unique Taxpayer Reference (UTR)
- Your National Insurance number
- Your P60 or P45, if you were employed during the tax year
- Records of income earned, such as invoices, bank statements, or sales records
- Details of any business expenses you plan to claim
- Interest statements from savings accounts
- Details of any dividend income
- Records of pension contributions or Gift Aid donations
Step 2: Complete the Main Form (SA100)
The SA100 is the core Self Assessment form, and everyone who files starts here. If you file online, HMRC asks a series of yes-or-no questions at the beginning. Your answers determine which sections you see, so you are only shown parts of the form that are relevant to your situation. The paper version, by contrast, includes every section regardless of your circumstances.
The SA100 covers areas such as employment income, pension income, savings interest, dividends, Gift Aid donations, student loan repayments, and Marriage Allowance claims. If your finances are relatively straightforward, this section may be all you need to complete.
Step 3: Check Whether You Need Supplementary Pages
Not everyone needs supplementary pages. HMRC requires them when you have specific types of income beyond standard employment or pension earnings, such as self-employment or property income.
The most commonly used supplementary pages include:
- SA103 for self-employment income
- SA105 for income from renting out UK property
- SA108 for reporting Capital Gains
These pages are also where you claim allowable expenses, which reduce your overall tax bill, so it’s worth taking your time here. If you file online, HMRC automatically prompts you to add the correct supplementary pages based on your earlier answers, which reduces the chance of missing something important.
Step 4: Review Everything Before You Submit
Go back through your entries before submitting. People often forget to claim expenses they’re entitled to, overlook pension contributions that qualify for tax relief, or miss recording Gift Aid donations. A careful review can genuinely lower your final bill.
Once you’re confident everything is accurate, submit the form. You’ll receive an automatic tax calculation right away, showing what you owe or, in some cases, what you’re due back.
Can You Amend a Self Assessment Form After Submitting It?
Yes, you can make changes to your Self Assessment tax return after you submit it. If you filed online, you’ll typically need to wait around 72 hours after submission before the system allows you to make amendments.
Should You Get an Accountant to Help?
You are not required to use an accountant to complete your Self Assessment form, and many people file it themselves without issue. That said, an accountant can be worth considering if:
- Your income comes from multiple sources, such as self-employment, property, and investments
- You want to make sure you’re claiming every allowable expense
- You’ve received a letter from HMRC about a previous return
- You simply want peace of mind that everything has been completed correctly
An accountant can also help you plan ahead, particularly if the Making Tax Digital rules are due to apply to you in an upcoming tax year.
Key Takeaways
- A Self Assessment form reports untaxed income to HMRC, including self-employment earnings, rental income, and investment gains.
- You need to register by 5 October if you’re filing for the first time, and file online by 31 January at the latest.
- Gathering your documents before you start makes the process far smoother.
- The SA100 is the main form, but you may also need supplementary pages depending on your income sources.
- You can amend a submitted return, though online changes usually require a short waiting period.
- An accountant can help if your tax situation is complex, but it isn’t a requirement for most straightforward returns.
Frequently Asked Questions
1. What happens if I miss the Self Assessment deadline?
You’ll receive an automatic £100 penalty, even if you don’t owe any tax. Further penalties apply the longer the return remains unfiled.
2. Do I need to file a Self Assessment form if I'm employed full-time?
Not usually, unless you have additional untaxed income, such as rental income or earnings from a side business above £1,000.
3. How long does it take to receive a Unique Taxpayer Reference (UTR)?
HMRC generally advises allowing around 20 working days after registration, though it can take longer during busy periods.
4. Can I submit a Self Assessment form on paper instead of online?
Yes, but the paper deadline is earlier, falling on 31 October rather than 31 January for online submissions.
5. What if I realize I made a mistake after submitting my return?
You can amend your Self Assessment return. If filed online, you’ll usually need to wait about 72 hours before the correction option becomes available.
