A Complete Guide to Your Self Assessment Form

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 when it’s your time to complete a Self Assessment form – you’re prepared.

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

If you’re still not sure, HMRC has a handy tool that can help you check online. If you’ve 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.

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.

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.

How do I fill out a 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). Here’s how to work through it.

Get your paperwork together first

When you complete a Self Assessment form, 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

Start with the SA100

The SA100 is the main Self Assessment tax return form, and it’s where everyone begins. 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 SA100 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.

Check if you need to fill in the supplementary pages

Not everyone needs to fill these in, 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.

Double check everything before you submit

It’s always worth going back through your figures before you hit submit. Common things people overlook include expenses they’re entitled to claim, pension contributions that qualify for tax relief, and Gift Aid donations. Getting this right could reduce your bill, so it pays to be thorough.

Once you’re happy everything 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.