Submitting a Self Assessment form can be a scary prospect – especially if you’ve never needed to submit one before. The good news is that it’s much less daunting when you know what to do, which documents you need to have, and when it needs to be done by.
This blog covers all of that for you, so you’ll be better prepared when it’s your time to complete your Self Assessment form.
What is a Self Assessment form?
A Self Assessment form is used to report your income and expenses to HMRC each tax year if you have untaxed income which needs to be declared. It’s HMRC’s way of making sure you’re paying the right amount of tax or, as sometimes happens, whether you’re due a refund.
Do I need to send a Self Assessment form?
If any of the following sound familiar then yes, HMRC requires you to complete a Self Assessment tax return form:
- You’re self-employed or a sole trader and this earns you more than the £1,000 trading allowance
- You’re a landlord with rental income
- You earn more than £1,000 from a side hustle or freelance work alongside your main job
- You’re a partner in a business partnership
- You’ve made money from investments or dividends
HMRC’s handy online checker will help if you’re still not sure.
Should I do a Self Assessment form or MTD Income Tax?
If you sent a Self Assessment tax return form in a previous year, then it’s also worth double-checking whether the MTD Income Tax rules apply to you; if they do, it will affect the way you report your income and expenses to HMRC. It’s a big subject, so we go over Self Assessment versus MTD in more detail in a separate article.
When do I need to register and complete my Self Assessment?
The deadline you need to hit depends on how you’re filing and what you’re doing.
| Action | What you need to do | Deadline |
| Register for Self Assessment | If you need to send a tax return for 2025/26 but didn't need to for the previous year, you should register for Self Assessment by this date. | 5th October 2026 |
| Submit a paper tax return | The deadline for submitting your Self Assessment tax return if you use a paper form. | Midnight, 31st October 2026 |
| Submit a tax return if you want to pay your bill through PAYE | Request that HMRC collect any tax you owe through your tax code. | Midnight, 30th December 2026 |
| Submit a tax return online | This is the deadline for submitting your tax return if you submit online. | Midnight, 31st January 2027 |
| Pay your Self Assessment tax bill | You should pay your tax bill in full by this date in order to avoid penalties. | Midnight, 31st January 2027 |
Missing any of the deadlines will trigger an automatic fine even if you don’t actually owe any tax, so it’s always worth getting it done in good time.
What to do before starting your Self Assessment form
If you’ve never filled out a Self Assessment form before, just looking at one can feel a bit overwhelming. Once you know what you’re looking at and have the right information ready, it’s actually pretty straightforward (we promise).
It’s best to get yourself fully prepared beforehand with all the documents you need, instead of adding to the stress if you can’t find bits and pieces in between.
You’ll want to have the following on hand before you start:
- Your Unique Taxpayer Reference (UTR)
- Your National Insurance number
- Your P60 or P45 if you worked for an employer during the tax year you’re submitting a form for
- Records of all income earned during the tax year, such as invoices, bank statements, and sales records
- Details of any business expenses you want to claim
- Savings interest statements from your bank or building society
- The details of any dividends from shares
- Details of pension contributions or Gift Aid donations made during the year
Which part of the Self Assessment form should I complete?
When you file online, HMRC will walk you through a series of yes/no questions at the start. These shape the form around your circumstances so you only see what’s relevant to you, but the paper form will show every section. The parts you need to complete depend on what sort of information you need to report for that particular tax year.
Start with the SA100
The SA100 is the main Self Assessment tax return form, and it’s where everyone begins. It covers things like employment income, pension income, savings interest, dividends, Gift Aid donations, and whether you’re repaying a student loan or claiming Marriage Allowance.
If your tax situation is fairly simple, this might be the only part you need to fill in. You can download it from the gov.uk website.
Check if you need to fill in the supplementary pages
Not everyone needs to send the supplementary pages with their Self Assessment form, and they’re primarily required for individuals reporting specific types of income and gains that go beyond basic employment or pensions. For example, property rentals and foreign income.
The most common supplementary pages are the SA103 for self-employment income, the SA105 for rental income from UK property, and the SA108 for Capital Gains. These pages are also where you declare any allowable expenses, which HMRC will deduct from your overall tax bill – so it’s worth being thorough.
If you’re filing online, the system will prompt you to add the right pages automatically based on your earlier answers, so it’s harder to miss anything.
| Form Name | Use It To Report… | Description |
| SA101 | Additional information | Report less common types of income such as compensation or royalty payments |
| SA102 | Income from employment or as a director | Use a separate form to tell HMRC about each source of income you receive as an employee or a company director. You won’t need to pay tax on it again if this has already been deducted through payroll. |
| SA103 | Self-employment income | Use a separate form to report each source of self-employed income. |
| SA104 | Partnership income | Tell HMRC about your share of the income as an individual partner in a partnership. |
| SA105 | Property income | Report income you receive from property, furnished lettings, or land in the UK. |
| SA106 | Foreign income or gains | Complete this form to tell HMRC about foreign income or gains, and to claim foreign tax credit relief. |
| SA108 | Capital Gains | Declare any Capital Gains you get in a tax year. |
| SA109 | Non-UK or dual resident | Record your tax residence status, and claim any allowances as a non-UK resident. |
Common errors to avoid on your Self Assessment form
It’s always worth going back through your figures before you hit submit. Some of the most frequent mistakes people make on their Self Assessment form include:
- Missing out on claiming allowable expenses they’re entitled to
- Claiming expenses when the total is below the £1,000 Trading Allowance (when it might be more beneficial to claim the allowance instead)
- Reporting payments they made into their employer’s pension scheme…
- …Or not declaring things like private pension contributions and Gift Aid donations (which qualify for tax relief)
- Missing out the ‘Number’ box on the page “what makes up your tax return”. You’ll need to enter the number of pages you’re completing for each section. For example, on question 1 which deals with employment, someone who worked for two different employers during the tax year will tick ‘Yes’ and put ‘2’ in the number box.
- Getting confused by the double-negative wording about how tax should be collected. Want HMRC to collect your tax through your employer or pension? Leave the box blank. Want to pay HMRC another way? Tick the box.
Once you’re happy your return looks correct, you’ll submit everything and receive your automatic tax calculation straight away.
Can I go back and make changes to my tax return form?
Yes you can amend your Self Assessment tax return form if you need to. You’ll normally need to wait 72 hours from when you submitted it before you can make any changes if you’re doing it online.
Can an accountant help me fill out a Self Assessment?
Absolutely – and for a lot of people, it’s well worth it. An accountant can take care of the entire process on your behalf, making sure your return is accurate, submitted on time, and that you’re not paying more tax than you need to.
If your tax affairs are straightforward, you might be comfortable doing it yourself. But if you’re self-employed, have multiple income streams, or just want peace of mind that it’s been done correctly, having an accountant in your corner makes a lot of sense.
Your dedicated tax accountant will help you avoid any nasty surprises in your Self Assessment tax return! Give us a call and talk to one of the team on 020 3355 4047, or get an instant online quote.
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