A Complete Guide To Self Assessment Tax Returns

When you’re a sole trader, keeping tabs on your money is hugely important. It’s how you know how much profit you’re making, how much to reinvest in your business and how much to pay yourself. But it’s also how you know how much tax you'll need to pay at the end of the year.

Your Coconut smart tax app may help you keep tabs on the money that flows in and out of the business, but your Self Assessment tax return is how you tell the people at HMRC (HM Revenue and Customs) how much tax you owe. And keeping things up to date means you can focus on the day job, rather than worrying about late-filing or late payment penalties.

So, in this guide, we'll cover everything you need to know about completing a tax return, so you know exactly what income you need to declare.

Before we start...

If you're feeling uncertain about your finances, we strongly recommend you work with an accountant. Having an accountant is valuable beyond just compliance and tax planning—it’s about having someone by your side with the experience to help you make the big decisions and build a better business.

Coconut also makes it really easy for you to collaborate with your accountant. Once you've invited yours through the app, they'll be able to view your business activity and bookkeeping data whenever they need to.

In this article:

What is a Self Assessment tax return?

Do I need to complete a Self Assessment tax return?

What income do I have to declare on my Self Assessment tax return?

How do I register for Self Assessment tax returns?

What information do I need to fill out my Self Assessment tax return form?

How do I fill out a Self Assessment tax return form?

What are the Self Assessment deadlines?

What are the penalties for missing the Self Assessment deadlines?

What if my circumstances change?

What if I make a mistake when filling in a Self Assessment tax return?

What are the penalties for mistakes when filing a Self Assessment tax return?

How do I work out my Self Assessment tax bill?

How do I pay my tax bill?

Can I pay my tax bill in instalments?

What if I can’t pay my tax bill?

What are CIS Tax Returns?

How do I claim a tax refund?

Where can I find more help & information?

What is a Self Assessment tax return?

Most workers don’t submit their own tax returns. That’s because they’re employees rather than self-employed, and they’re paid via PAYE (Pay As You Earn). Each week or month, their wages will have income tax, National Insurance (NI), pension payments and other deductions removed from their wage at source, so the money that arrives in their bank account is theirs to keep.

It’s a different story if you’re a sole trader, have multiple sources of income (for example, you work a day job but also receive rental income from property you own), or have made a profit on an asset you’ve sold (capital gains). The income tax and NI you owe won’t be deducted automatically, because HMRC won’t know how much you owe them. That’s the point of a Self Assessment tax return.

A tax return is a form you’ll need to submit to HMRC every year to declare the money you earn that isn’t paid via PAYE. Most self-employed people submit the form online. It is possible to submit a paper version, too.

You and/or HMRC use the information on your tax return to calculate your overall tax bill. If you’re lucky, HMRC might occasionally determine that you’re actually due a tax refund. Even if you pay tax through PAYE because you’re employed in one job, you’ll still need to complete a tax return if you receive money from other sources like self-employment.

The Self Assessment tax return is a legal requirement. You don’t have the option to quietly trade without mentioning it to HMRC. There are strict deadlines for submitting your Self Assessment tax return and paying any tax you owe, and there are penalties if you miss those deadlines. All of which can make Self Assessment seem rather scary. But it doesn’t have to be.

That’s why we’ve put this guide together, to help you prepare, organise and understand what you’ll be asked for, so filing your tax return is easy.

Do I need to complete a Self Assessment tax return?

If you have any income that isn’t taxed at source, chances are you’ll need to complete a Self Assessment tax return. If you’re a sole trader, in a business partnership or a company director (because as a director you’ll pay personal tax as well as the tax your company owes), you’ll definitely need to complete one.

You’ll also need to submit a return if HMRC asks you to. They’ll do this via a ‘notice to file’, which is the ominous looking letter that always arrives in a brown envelope.

There are other reasons you might be asked to file a tax return and we cover many of them in our article, “Do I need to complete a Self Assessment tax return?”

What income do I have to declare on my Self Assessment tax return?

Your tax return contains your declared income, but what income do you need to declare? You’ll need to complete a Self Assessment tax return if, in the last tax year…

• Your annual income was more than £100,000, even if you only received that money from paid employment (that is, via PAYE) or from pensions
• You earned £10,000 or more from savings and investments
• You earned £2,500 or more in untaxed income, savings or investments. That would include, for example, tips or commission that wasn’t included in your PAYE
• You received £10,000 or more from property (before deducting allowable expenses) or £2,500 or more from property (after deducting allowable expenses)
• You had income from trusts, settlements or estates
• You had income from abroad you need to pay tax on, or you live abroad but had an income in the UK
• You were a trustee of a trust or registered pension scheme
• You were a business partner or a director of a limited company (unless it was a non-profit organisation, such as a charity)
• Your State Pension was your only source of income, but higher than your personal tax allowance
• You made capital gains on profits from selling things like shares or a second home (in which case you may need to pay Capital Gains Tax)
• You or your partner received Child Benefit and your income was over £50,000

If you earned over £50,000 in the 2020/21 tax year (the amount may change over time so double check that threshold if you’re reading this post-5th April 2023) and you’ve made extra pension contributions, you might have to complete a Self Assessment to claim back the extra tax relief you’ll be owed.

You can also fill in a Self Assessment tax return if you want to make voluntary NI contributions. You might do this if your State Pension forecast indicates that you won’t qualify for the full State Pension when you retire. Making voluntary contributions will top up your NI payments and ensure you qualify for benefits at the full rate.

Unless you earn more than £100,000, if the only income you earn is as an employee via PAYE you wouldn’t usually need to fill in a Self Assessment tax return.

How do I register for Self Assessment tax returns?

Before you submit your tax return you’ll need to register for Self Assessment with HMRC as one of the following:

• Self-employed or a sole trader (more information on how how to register as self-employed)
• Not self-employed (but earning untaxed income)
• A landlord (receive income from land and property)
• A company director
• A partner or business partnership

When registering, you’ll want to make sure that all the personal information HMRC holds about you is up to date. You can do this via your personal account.

What information do I need to register with HMRC?

• Your name
• Address
• Previous address (if you've lived in your current address for less than 3 years)
• National Insurance number
• Date of birth
• Phone number
• Email address
• Date you started as self-employed
• Nature of your business
• Business address
• Business telephone number
  

If you have your 10-digit Unique Taxpayer Reference (because you’ve previously completed a tax return) you’ll need to give that too.

When do I need to register with HMRC?

Rest easy. HMRC won’t hound you for tax the moment you think of a new business idea. You have a ‘grace’ period for registration, so you’ll only need to register by 5th October in your business' second tax year.

The tax year runs from 6th April to 5th April. So if, for example, you started your self-employment on 1st May 2022, you've got until 5th October 2023 to register.

Bear in mind though, that although you’ll only need to register with and submit a return to HMRC in your second year, the tax you pay will be calculated from the moment you start trading. To avoid being caught out, and as a (very) general rule, put one third of your profit into a savings account, so you know you’ve got the money to pay your tax bill when it falls due. 

For a more personal and accurate understanding of how much money to set aside for tax, download the Coconut app. It removes the guesswork because it’s calculated on your income and expenses.

The easiest way to register for Self Assessment is by doing it online. Here's how:

If you’ve never registered before

To get started, complete the following three steps:

• Visit the HMRC Sign In page
• Click on the green ‘Sign In’ button, then click on ‘Create Sign In Details’
• Enter your email address

This will generate your Government Gateway account User ID which will be emailed to you. You’ll use this to access lots of government services online, including Self Assessment.

You’ll also get a Unique Taxpayer Reference number (UTR) in the post. It should take around 14 days to arrive, but that could be 21 days if your delivery address is outside the UK. You’ll use the UTR to file your return and you’ll use the same UTR year after year, so keep it safe when you receive it.

Your Government Gateway activation code will arrive by separate letter. You need the code to log into your account for the first time and it expires after 28 days, so best complete your registration as soon as possible to be on the safe side.

Most people complete registration online but you can do it by post if you prefer using form SA1. Download it online, then print and post.

If you’ve registered in the past

If you have registered in the past you’ll need to re-register, and for that you’ll need the 10-digit Unique Taxpayer Reference number that allows you to manage all your personal tax affairs online. If you’ve ever completed a tax return, you’ll find the UTR on that.

What information do I need to fill out my Self Assessment tax return form?

To complete your tax return, you’ll need to gather together all of this:

• NI Number
• Unique Taxpayer Reference
• Government Gateway User ID
• Self-employment income (Invoices, bank statements)
• Self-employment expenses (Receipts, bank statements)
• Details of any property income or expenses
• Tenancy agreements (if appropriate)
• Details of income or interest from savings or investments (Savings account or investment company statements) (Form P60 (given annually by your last employer); P45 (given to you by your employer when you leave); P11D (a form your employer would have used to tell HMRC about any ‘benefits in kind’ (eg company car) you received)
• Details of any chargeable capital gains made during the year (that is, any personal assets you’ve sold—or otherwise disposed of—worth more than £6,000, except your car). That could include cryptocurrency.
• Details of any income you’ve had from any employment (as opposed to self-employment) or pensions
• Details of any income from partnerships
• Details of charitable donations and Gift Aid
• Details of pension contributions
• Redundancy payments/employment benefits

It’s important to keep your records safe. You don’t need to send original documents to HMRC, but they can ask to see them for six years from the 31st Jan Self Assessment deadline. If your records are digital, you’ll find having them all in a folder for each year saves an awful lot of panic should HMRC start asking questions down the line. Keeping your paperwork in order (and all in one place) gets much easier when you use Coconut.

Keeping things organised is also an indicator to HMRC that you’ve taken reasonable care to get your return right.

If any of your records are on paper (or an Excel spreadsheet) now’s the time to start getting things ready for quarterly tax returns with Making Tax Digital. If you’re self-employed and pay VAT, you should already be doing that. If you don’t pay VAT, you’ll need to ensure that all your records are digital by 2026. For more, take a look at Coconut’s Guide to Making Tax Digital.

Do I need a UTR to be self-employed?

Your UTR number (you might hear it called your ‘tax reference number’) is unique to either you or your company. It’s how HMRC identifies you for anything tax related. You don’t need it to begin trading as a self-employed worker, but you will need it to complete your Self Assessment tax return.

Where can I find my UTR?

Your UTR is 10 digits long and you’ll find it on any previous tax returns and other documents from HMRC, including:

• Your ‘Welcome to Self Assessment’ letter (SA250)
• Any ‘notice to file a tax return’ letters
• Your HMRC statement of account
• Payment reminders

If you’ve already registered for HMRC’s online services you should be able to log in and find your UTR number there. Alternatively, call the Self Assessment helpline.

How do I fill out a Self Assessment tax return form?

When you submit your tax return you’ll only need to complete the parts relevant to you. 

The return is in two parts. The main section (the SA100) deals with income you received from employment, pensions, charitable donations, allowances and benefits. The second part—the supplementary pages—are where you’ll declare income from self-employment, property, capital gains, company directorship, or foreign income.

Our seamless integration with GoSimpleTax, means you can effortlessly transfer your data from Coconut directly into GoSimpleTax. This streamlined process ensures that your tax return is populated accurately and promptly, ready for submission directly to HMRC.

What are the Self Assessment deadlines?

Tax returns relate to tax years, not calendar years. Here are the important dates to remember:

Find more information about the key Self Assessment deadlines.

As one tax year ends, your online tax return becomes due (although you can submit it at any point up to and including 31st January of the following year). You’ll need to pay any tax owed by the following 31st January too.

For example, the 2021-22 tax year ended on 5th April, 2022. You’ll need to have submitted your tax return and paid any money you owe by 31st January 2023.

For most people, these deadlines are fixed. In some circumstances, however, where HMRC sends you a letter telling you to complete a tax return after 31st October, you may be given a later deadline, usually three months from the date of the letter.

If you owe less than £3,000 for the year and you receive additional income through PAYE or pensions, you can choose to pay your tax bill through an adjustment of your tax code. HMRC will collect your tax over the following year through your wages or pension. If you want to choose this option, you’ll need to submit a paper return by 31st October or an online return by 30th December.

What are the penalties for missing the Self Assessment deadlines?

If you file your tax return late or fail to pay any money you owe on time, you may face penalties. If you don’t file on time and you also pay late, you’ll face penalties for both, so things can really start to spiral fast. If you think you’ll struggle to pay your tax return, it’s important to talk to HMRC as soon as possible (see below).

Although HMRC will accept some reasonable excuses for late filing of your tax return, the list isn’t a long one. 

If you are given a penalty, you can appeal if you have a reasonable excuse. Appeals are assessed on a case-by-case basis so there’s no guarantee that your penalty will be waived. You should never submit your tax return late on the assumption that an appeal will be successful.

If you do need to file late, let HMRC know as soon as you can. Showing them that you’re doing your best to put things is always a good move.

Late filing penalties

Penalties for missing the 31st January tax return filing deadline increase over time:

Late payment penalties

Penalties for late payment also increase the longer you take to pay:

Interest on late tax payments

In addition to the late payment penalties above, HMRC will charge you interest if you don't pay your tax by 31st January. You’ll need to pay 2.75% interest on late payments from 1st February, rising to 3% from the end of February. You’ll be charged interest on the unpaid tax AND unpaid penalties.

What counts as a ‘reasonable excuse’ to miss the deadline?

“I’m not a paperwork person. I always got my sister to do my return but we fell out.”

“I left my return in the shed and a rat ate it.”

“I had an argument with my wife and went to Italy for 5 years.” 

That last one’s our favourite.

In case you’re wondering, these are genuine excuses given to HMRC. And as you might expect, HMRC wasn’t impressed. But there are reasons HMRC will accept and which could help you avoid a late filing or payment penalty.

Typically, these involve the unforeseeable, the unexpected and the 'beyond your reasonable control'. HMRC lists these as including:

• A failure in the HMRC computer system
• Your computer breaks down while you’re in the middle of your online return
• A serious illness, disability or serious mental health condition has made you incapable of filing your tax return
• You registered for HMRC Online Services but didn't get your activation code in time
• A fire, flood or theft prevented you from completing your tax return
• Postal delays that you couldn’t have predicted
• Delays related to a disability you have

HMRC considers each case on merit and still expects you to have done everything in your power to submit your return on time. It won’t accept any excuse where you haven't made a reasonable effort to meet the deadline. Examples might include where you…

• Found the online system too complicated to follow
• Were let down by your accountant
• Forgot about the deadline (even if you didn’t get a reminder from HMRC)
• Didn’t try to re-submit your return on time once yours or HMRC’s IT issues were put right
• Registered for HMRC Online Services after the filing deadline

HMRC will expect you to be up-front and co-operative. You can read more about penalties for failure to notify on the GOV.UK website.

What if my circumstances change?

HMRC may work to tax years but life is never quite so regimented. So what happens if your circumstances change during the tax year?

If you start earning untaxed income (that is, any income not taxed before you receive it through PAYE), you must let HMRC know by 5th October following the end of the current tax year. HMRC will then decide whether you need to complete a tax return.

Reasons you might start earning untaxed income could include starting a new self-employed job, starting to receive rental income on a property you’ve been refurbishing, or starting to receive commission from a new job.

If you know you’ll earn less this year than you did last (so this year’s tax bill will be lower), you can ask HMRC to reduce your payments on account. ‘Payments on account’ are essentially advance payments towards your tax bill and we explore them more below.

What if I make a mistake when filling in a Self Assessment tax return?

You will be able to look back at your tax return and correct any errors before you officially submit it. Once you’ve submitted it, you can still make changes up until the Self Assessment deadline. So for the 2021-22 tax year, you have until 31st January 2023 to make corrections to your online return. This is another reason why doing your return online (rather than paper) makes sense—you get longer to make changes, you can make amends instantly and you can adjust your payments on account.

Naturally, the earlier you submit your return, the more chance you’ll have to review it and spot any errors.

If you spot an error after the deadline you’ll need to write to HMRC about any changes.

What are the penalties for mistakes when filing a Self Assessment tax return?

HMRC may penalise you for making mistakes with your tax return, although whether they do and how much will depend on the nature of the mistake:

How do I work out my Self Assessment tax bill?

HMRC will calculate your tax bill for you, but it's wise to keep on top of your income and expenses so that you can ensure you’ve set aside enough money for tax throughout the year. Keeping track is much easier with an accounting and tax app like Coconut.

Your final tax bill will be the difference between the amount you have calculated and the payments on account you’ve already made (see below). This is known as the ‘balancing payment’.

With an online return, you’ll be able to see the exact amount you owe even before you press submit. For paper tax returns, HMRC will work out how much you owe and send you the tax calculation by post.

In the past, it’s often only been at this part of the process when sole traders have discovered how much tax they owe. But with Coconut, our tax return software helps you prepare your tax return as you go. So not only does your end of year Self Assessment tax return take minutes rather than days; you never get any nasty surprises. 

How do I pay my tax bill?

How much tax you owe depends on the income tax band you're in. HMRC works this out using the information you give in your Self Assessment form.

Tax bands change from year to year. There’s a level at which you pay no tax, because your earnings are below the ‘Personal Allowance’ (that is, the amount the Government says you’re allowed to earn before it starts taxing you). Then there’s a basic rate of (currently) 20% which you pay on income above the Personal Allowance threshold. Two further bandings of 40% and 45% exist for higher income earners. 

Here's a full guide to the current self-employed tax rates.

You’ll also need to pay NI as part of your final tax bill. Sole traders will usually pay two rates:

• Class 2 for profits of £6,725 or more a year
• Class 4 for profits of £9,881 or more a year

The deadline for paying your tax is 31st January following the end of the last tax year (the same date as the deadline for submitting your online tax return). So, for the tax year April 2021 – April 2022, the deadline for submitting your return will be 31st January 2023.

There are lots of ways to pay your tax bill, including debit card, telephone banking, direct debit (allow 3-5 working days for the direct debit to be processed), at your bank using the Clearing House Automated Payment System (CHAPS), BACS Direct Credit, or cheque. 

Some methods take longer than others, so you'll have to account for this to ensure you don't miss the deadline.

Paying Self Assessment online

The big advantage of paying the tax you owe through your online bank account is that payments reach HMRC instantly (or near-instantly). If the deadline is fast approaching, paying online is a way to ensure you meet it.

You can pay your Self Assessment tax online here. You can use any debit card but you won’t be able to use a personal credit card. 

Paying Self Assessment by cheque

It’s possible to pay your tax bill by cheque, although we wouldn’t really recommend it. There’s always the risk that the cheque will go missing or simply won’t get processed—issues you don’t face when you pay online. 

If you do want to pay by cheque, take it to your bank, building society or post office. Alternatively, you can post it to HMRC at HMRC, Direct, BX5 5BD.

Make the cheque payable to ‘HM Revenue & Customs only’ and write your UTR on the back. Although your UTR is a 10-digit number, some payment methods require an 11-digit reference, which is your UTR followed by the letter ‘K’. Add the K to your reference when paying by cheque, BACS, CHAPS or direct debit.

You’ll also need to include a printed version of your Self Assessment payment slip. You can get one here.

Allow three working days for your payment to reach HMRC. 

Can I pay my tax bill in instalments?

Yes, you can make payments in instalments. These are an advance on your next tax bill; they can’t be used to pay last year’s bill (so you’ll need to be up-to-date with your previous Self Assessment payments).

You can arrange a ‘budget payment plan’ through your online account and decide how much you want to pay each week or month. You’ll need to have this arrangement in place before the filing and payment deadline. 

You can also choose to stop paying for up to six months if you need to.

What are ‘Payments on Account’?

Payments on account are essentially advance payments towards your tax bill. They help self-employed people spread the load of a tax bill across the year instead of paying it all in one lump sum.

You’ll make two payments on account each year, unless your last Self Assessment tax bill was less than £1,000 or you've already paid over 80% of the tax you owe.

Each payment is half of your previous year's tax bill. Payments are due by midnight on 31st January and 31st July.

As the exact amount of tax you pay this year is unlikely to be identical to the amount you paid last year, you’ll need to make up any outstanding remainder (known as a ‘balancing payment’) by midnight on 31st January the following year.

Example: Your tax bill for the 2020/21 tax year is £5,000. The previous year you made two payments on account of £2,000.

By 31st January 2022 you should have paid £3,500. This includes the £1,000 balancing payment for your 2020/21 tax bill and your first payment on account for your 2020/21 tax bill (£2,500).

Your tax payments will also include NI contributions (NIC) if you earn over the NIC threshold.

Payments on account do not include anything you owe on a student loan.

If it turns out that you’ve paid too much on account, HMRC will send you a refund.

If you know next year’s tax bill will be lower than this (because you’ve earned less) you can reduce your payments on account, but don’t ever be tempted to forecast a lower amount simply to pay less tax. If you end up underpaying, you’ll be charged interest on the shortfall.

For more information, read our guide to payments on account.

What if I can’t pay my tax bill?

If you can’t afford to pay your tax bill, contact HMRC’s Business Payment Support Service on 0300 200 3835. The sooner you call them the better.

Depending on your personal circumstances and what you can afford to pay, HMRC may give you more time to pay, or they may allow you to spread payments over a few months. This is called a ‘Time to Pay arrangement’. In deciding how much flexibility to offer, HMRC will consider:

• How much you owe
• Income
• Expenditure
• Assets
• Savings and investments

Although you may still incur interest on the money you owe, you won’t pay penalties if you agree a Time to Pay arrangement. If you don’t stick to the agreed plan, however, you will face penalties.

If you don’t think you’ll be able to submit your tax return for mental or physical health reasons, or because you have experienced abuse or trauma, contact HMRC for advice on 0300 200 3310.

Find out more about what to do if you can’t pay your tax bill at GOV.UK

What are CIS Tax Returns?

If you’re a self-employed subcontractor in the building trade, then in addition to registering for Self Assessment, you’ll also have to register for the Construction Industry Scheme (CIS).

The CIS was set up in 1999 by the Treasury because of what it saw as a major problem in the way contractors were accounted for for tax purposes. It means that, when you're paid through CIS, your contractor chisels off a hefty chunk of your pay before you get it.

This goes straight to HMRC as a sort of ‘advance payment’ of income tax and NI. It means construction is the only sector where self-employed people operate in a weird sort of hybrid PAYE scheme.

For more information, read our full guide to the Construction Industry Scheme.

How do I claim a tax refund?

If you’re due a refund on your tax return (and providing you’ve entered your bank details on the return) then HMRC should automatically refund any money you’re owed around two weeks after submission and processing.

If, at the end of the financial year, you’ve overpaid income tax for some other reason (e.g. you paid too much tax on income from another job or on foreign income during the financial year) you can ask for a refund.

Claim your tax refund online and you should receive your money in five working days. Or HMRC will send you what’s known as a P800 tax calculation showing the overpayment, and you’ll automatically receive a cheque in the post. That should take 14 days.

Where can I find more help & information?

There are lots of ways to get help with completing your Self Assessment tax return.

As a starting point (and because contacting HMRC can often take time), you can find additional guidance on Self Assessment tax returns in Coconut’s other guides:

• How to submit your Self Assessment Tax Return in half the time
• Get the right numbers for your tax return using Coconut
• The complete guide to self-employed tax rates for the current tax year
• Making Tax Digital: what you need to know about quarterly tax returns

If you have more specific questions about your own Self Assessment tax return, you should get in contact with HMRC: