Everything you need to know about Self Assessment tax returns


If you’re self-employed or have income streams in addition to employment wages, you may have to tell HM Revenue and Customs (HMRC). Once reported, HMRC will decide if you need to pay income tax on these earnings. This is done through a Self Assessment tax return.

Filing your own tax returns for the first time is a big deal, as all of the responsibility to get it right falls on you. Fortunately, the process of completing a Self Assessment tax return is relatively straightforward.

In this guide, we’ll take you through everything you need to know about Self Assessment, covering the what, why, when and how to make filing a tax return stress-free.

Table of contents

What is a Self Assessment tax return?

Self Assessment is a process HMRC uses to collect income tax from certain people whose income isn’t taxed automatically when it’s paid. This includes business owners and people who earn other income outside of a full-time job.

Where employed workers generally have their tax automatically deducted by their employer through the Pay As You Earn (PAYE) scheme, paying tax and National Insurance on income outside of that scheme is down to the individual.

Self Assessment can be completed via an online or paper form and requires you to give details on all of your income and expenses for the previous tax year. From there, HMRC will work out if you have to pay tax on your profits and how much is owed.

Top Tip: Getting your finances in order as a startup can be an overwhelming task. It’s important to become acquainted with the fundamental accounting steps and to familiarise yourself with how an accountant can help. To learn more, read our complete guide to accounting for startups 🧮

Do I need to complete a Self Assessment tax return?

According to HMRC, you’ll need to complete a Self Assessment tax return if, in the last tax year, you:

  • Earned more than £1,000 as a self-employed sole trader
  • Were a partner in a business partnership

You may also need to send in a Self Assessment tax return if, in the last tax year, you:

  • Received COVID-19 grants or support payments
  • Earned money from renting a property
  • Earned tips and commission
  • Earned income from savings, investments and dividends
  • Can claim some Income Tax relief
  • Want to prove you’re self-employed, for example to receive Maternity Allowance or claim Tax-Free Childcare
  • Received Child Benefit and your income, or your partner’s, went over £50,000 (you’ll need to pay a high income charge)
  • Have income from overseas that you need to pay tax on*
  • Lived abroad but have taxable income from the UK

You can check with HMRC to see if you need to complete a Self Assessment tax return.

*If you’re a UK resident with foreign income or capital gains, you must include that information on your Self Assessment tax return. That said, there is some foreign income that’s taxed differently. If all of the following scenarios apply, you do not need to include foreign income on your Self Assessment return:

  • Your only foreign income is dividends
  • Your total dividends – including UK dividends – are less than the £2,000 dividend allowance
  • You have no other income to report

Different rules may apply if you permanently live abroad. If that is the case, you will be considered a ‘Non-domiciled’ resident and you may not need to pay any UK tax on foreign income.

Top Tip: The tax year runs from 6th April until 5th April. But, in your first year of business, depending on your incorporation date and other various factors, your tax year and financial year may slightly differ. To learn more about the nuances of the tax year and how it affects your tax return, read our simple guide to small business tax 💷

Your pension status may also require you to complete a tax return if you:

  • Are a trustee of a registered pension scheme or trust
  • Received an income from your State Pension, any private pensions, taxable benefits or other earnings from employment or investments that totalled more than your personal allowance 
  • If your private pensions total more than £1,073,100

What’s classed as trading?

If you occasionally sell items on websites like Gumtree or eBay or in classified ads, HMRC doesn’t class this as trading and any money you’ve received from selling these goods won’t be subject to tax or require you to complete a Self Assessment tax return.

If, however, you sell items more frequently and earn a regular income from doing so, HMRC may view this as trading.

To help you (and HMRC) work out whether or not you are trading, HMRC has put together a list of ‘badges of trade’. These are:

  1. Profit-seeking motive. Acquiring an asset to sell it at a profit. For example, buying goods in bulk at a wholesale price and selling them individually at a higher price.
  2. Transactions. Repeated transactions may indicate trading.
  3. Nature of asset. HMRC looks for what is being sold. For example, if you’re selling a used pair of shoes that you were fond of, this points to the sale not being a trade. If, though, you’re selling mobile phone cases that never gave you enjoyment and you simply want the money for, this does look like trading.
  4. Connection with existing trade. If what you’re selling is linked to a current trade, HMRC will see it as part of that trade. For example, if you have an eBay store selling mobile phone cases and then you start to sell screen protectors, the two would be treated as one by HMRC.
  5. Modification of asset. If you’ve taken an item and repaired, changed or improved it to make a profit, HMRC may see this as trading.
  6. Organisation of the activity. Whether you sold the item in a way that’s consistent with a trading business or to raise funds to cover something like a family emergency.
  7. Method of finance. If you purchased goods with a loan or borrowed money that could only be paid back from the sale of the goods, HMRC may see this as trading.
  8. Interval of time between purchase and sale. If you’re purchasing goods and selling them on quickly, this points to the sale being made for trading.
  9. Supervening trade. If the item you sold was inherited or given to you as a gift, it’s unlikely that you’re trading.

How do I register for a Self Assessment tax return?

Self Assessment tax return registration page on the GOV.UK website
HMRC Self Assessment online registration (source: GoSimpleTax)

If you’ve never submitted a tax return before, you’ll need to register for Self Assessment. You must register by 5th October after the end of the tax year that you are required to submit a tax return. 

For example, if you need to submit a tax return for the 2021/22 tax year, you’ll need to register by 5th October 2022. If you miss this deadline for your Self Assessment tax return, you may have to pay a penalty. We’ll discuss the possible penalties in detail in a further section.

Once you’re registered, HMRC will send you a Unique Taxpayer Reference (UTR) along with instructions on how to create a Government Gateway account. With your Government Gateway ID, you can access the Self Assessment area to submit your tax return.

Top Tip: You don’t have to manage Self Assessment alone. Hiring a professional accountant can be a good idea, as they are experts in Self Assessment tax returns and can help you register properly and ensure you never miss a deadline. To learn more about how to find the right accountant for you, read our guide on how to choose an accountant for your small business 🤝

When are the Self Assessment tax return deadlines?

To avoid getting hit with a penalty fee, HMRC must receive your tax return and any money you owe by the deadline.

Along with the 5th October deadline for registering for Self Assessment, HMRC has the following deadlines:

  • Paper tax returns must reach HMRC by midnight 31st October after the end of the tax year
  • Online tax returns must be submitted by midnight 31st January after the end of the tax year
  • Any tax owed must be paid by midnight 31st January after the end of the tax year

If you’re a PAYE employee, you can pay your Self Assessment bill through your PAYE tax code (HMRC can automatically collect it from your paycheque) if the following apply:

  • you owe less than £3,000 (part payments do not count)
  • you already pay tax through PAYE (eg, you’re employed or get a company pension)
  • you submitted your paper tax return by 31st October or your online tax return online by 30th December

If you’re a trustee of a registered pension scheme or a non-resident company, tax returns must be completed using paper forms and submitted by 31st January after the end of the tax year.

If you’re in a business partnership where your partner is a limited company and they have an accounting date between 1st February and 5th April, paper returns must be submitted 9 months after the accounting date and online returns 12 months after the accounting date.

The cost of missing a deadline

If you file your tax return up to 3 months late, you’ll be hit with a penalty of £100. You’ll have to pay more if it’s even later, or if you’re late paying the tax you owe. Interest will also be charged on late payments.

You can estimate the cost of a penalty on the HMRC website.

If you’ve got a genuine reason for submitting a return late, contact HMRC as soon as possible. If you have missed your Self Assessment payment date you can phone the Self Assessment Payment Helpline on 0300 200 3822.

You can appeal a late filing penalty if you have a reasonable excuse. HMRC list these excuses as:

  • The death of a partner or close relative shortly before the deadline
  • An unexpected stay in hospital that prevented you from submitting your tax return
  • You had a serious or life-threatening illness
  • Your computer or software failed while you were preparing a return
  • Issues with HMRC’s online services
  • A fire, flood or theft prevented you from submitting your tax return
  • Unforeseen postal delays
  • Delays related to disability
  • A person you were relying on to complete your tax return didn’t do it

The following excuses, however, won’t be accepted:

  • Your cheque bounced or payment failed
  • You found the online system too difficult
  • You didn’t receive a reminder from HMRC
  • You made a mistake on your tax return

What information will I need to complete a Self Assessment tax return?

Image of Tide member using the web and mobile app
Easy finance admin on web and mobile with Tide (source: #wocintechchat, edit Tide)

Before you sit down to complete your tax return, make sure you have everything you need on hand. This way the process will be straightforward.

Here’s what you’ll need:

  • Your 10-digit UTR
  • Your National Insurance Number
  • Details of your self-employment income
  • Details of any self-employment business expenses
  • Details of any pension or charitable contributions that may be eligible for tax relief
  • Details of property rental income

If you earn an income through employment, you’ll also need:

  • A P60 from your employer showing your income and the tax you have already paid
  • A P9D or P11D showing any benefits or expenses you received
  • A P45 if you have left a job during the tax year

Top Tip: It’s important to organise your finances as soon as possible after you launch your small business. Knowing what you can and can’t claim as expenses will make things easier when it comes to filing taxes. To learn more, read our guide on Self Assessment expenses 💷

How to complete your Self Assessment tax return

As long as you have accurate records of your income and expenses, filling in the tax return online should be no more daunting than completing any regular expense form. All you need to do is enter the information and the system will take care of the rest.

The tax return has a main section (SA100) and supplementary sections for income from other sources that you haven’t paid tax on.

Filling in the SA100 Income section

The SA100 section covers income from sources other than employment or self-employment, along with any pension or charitable contributions you’ve made. This includes:

  • Interest and dividends. Details of taxed and untaxed interest from UK banks and building societies, dividends from UK companies, plus foreign interest and dividends.
  • Pension, annuities and state benefits. Details on the total and gross amounts of any State Pension, the gross amount of any other pension lump sums or annuities and details on any benefits, including Jobseeker’s Allowance, Incapacity benefit, Carer’s Allowance, Industrial Death Benefit and Bereavement Allowance.
  • Blind Person’s Allowance. You must note whether or not you’re claiming this.
  • Student loan repayments. You must note whether or not you’re repaying a student loan and details of deductions made by your employer.  
  • High-income Child Benefit charge. If you earn over £50,000 and receive Child Benefit, you’ll need to complete this section.
  • Marriage Allowance. If your income for the tax year was less than the Personal Allowance, you can transfer the remaining allowance to your spouse.
  • Pension contributions. Details on all payments made into a registered pension scheme or annuity contract where contributions were made after tax.
  • Charitable donations. Details of all Gift Aid donations.

Filling in the Supplementary tax return pages

If you’re self-employed, have income from property or have capital gains to declare, you’ll need to complete the necessary supplementary pages.

These additional Self Assessment forms are:

  • Self-employed (SA103 form)
  • UK property (SA105 form)
  • Capital Gains (SA108 form)

Each of these forms is split into two sections: income and expenses.

For self-employment (SA103)

Your income is everything you’ve earned through self-employment during the tax year, before expenses. If you have other incomes from self-employment, you’ll need to enter the one you earn the most from as your main income.

For example, if you work as a freelance web designer and earned £20,000 through selling your services, but also earned £8,000 through an eBay store selling crafts, you should list your web designer income as your primary income.

Your expenses are anything you’ve spent money on for your business in the last tax year. If you earned below £85,000 during the tax year your expenses can be entered as a total sum. If you earned over £85,000 you’ll need to list them individually.

If you’re a sole trader and claim the trading allowance of £1,000, you won’t be able to claim expenses.

The trading allowance is an automatic allowance that can be used against trading, casual or miscellaneous income to exempt it from tax. For example, if you had a total income of £1,500 through selling artwork at a local monthly craft fair and you claimed the trading allowance, your taxable profit would be reduced to £500.

You can find out more about the trading allowance on the HMRC website.

What expenses can I claim if I’m self-employed?

  • Travel costs such as fuel, train and bus fares, and parking costs
  • Office costs such as a phone bill, internet and stationery
  • Clothing such as a company uniform
  • Equipment costs such as tools and computers
  • Stock costs including materials
  • Business premises costs such as heating, lighting and business rates
  • Staff costs such as salaries, wages and other staff expenses
  • Advertising and marketing costs such as a website or flyers
  • Financial costs such as bank, loan or credit card charges
  • Professional service costs such as legal advice or accountancy services
  • Training costs such as courses that helped you improve the skills and knowledge you use in your business

You can view the full list of allowable expenses on GOV.UK.  

Track expenses automatically with Tide Expense Cards. Upload receipts, categorise payments, set spending limits and keep an eye on business spending – all in the Tide app.

expense cards - smart company expenses

For UK property (SA105)

Your income is the total amount you’ve earned from any rental properties, holiday lettings and land leasing (including any premiums earned).

Expenses can be claimed on the costs of owning and maintaining property, unless you claim the trading allowance.

What expenses can I claim if I’m a landlord?

  • Building repairs or maintenance costs (for example, if you hired a plumber to fix a shower)
  • Rate costs such as council tax, gas and electricity
  • Financial costs such as interest on loans, insurance and ground rent
  • Professional service costs such as legal advice or accountancy services

You can view the full list of allowable property letting expenses on GOV.UK.

For capital gains (SA108)

Capital gains tax is charged when you sell, give away, or otherwise dispose of an asset for profit. Assets include things you own such as property, antiques, or cryptoassets.

Expenses can be claimed for allowable costs on buying and improving assets.

What expenses can I claim on capital gains?

  • The price paid when the asset was purchased
  • Stamp duty paid when buying the property
  • Improvement costs, as long as the improvements are reflected in the asset at the time of disposal

You can view the full list of allowable expenses in HMRC’s Capital Gains Manual.

According to GOV.UK, to report any Capital Gains Tax over your annual allowance, you’ll need:

  • Calculations for each capital gain or loss you report
  • Details of how much you bought and sold the asset for
  • The dates when you took ownership and disposed of the asset
  • Any other relevant details, such as the costs of disposing of the asset and any tax reliefs you’re entitled to

You must report any Capital Gains Tax on a UK property within 60 days of selling it if you completed the sale after 27 October 2021. Failure to do so may result in paying interest or a penalty. 

If you’re not a resident in the UK, you must report sales of UK property as a non-resident, even if you have no tax to pay.

Once you’ve entered all of the information, hit “Submit” and you’re all done. HMRC will then calculate if you owe tax, how much tax and what National Insurance contributions you’ll need to pay.

Top tips for filing your Self Assessment Tax Return

  • Get registered for Self Assessment as soon as possible. Don’t leave it too close to the 5th October deadline. HMRC says it can take up to 10 working days to receive your UTR number and up to 10 additional days to receive an activation code. So look to register at least one month before the deadline.
  • Download HMRC’s free help sheets to assist you with each section of the tax return. If you’re completing your return online, look out for the “?” icon next to each of the fields. This will give you additional information.
  • Don’t rush it. You don’t need to complete your Self Assessment return in one go. You can save your progress and come back to it. With that in mind, start your tax return as soon as possible. This will give you the time you need to gather actual figures where you’ve entered estimates and check that all information is complete.
  • Keep accurate records. Around one in 20 tax returns is subject to further investigation. If your return is chosen for investigation, you’ll need to produce records to show that the figures are correct. Don’t throw anything away. As a taxpayer, you need to keep records for at least a year after filing. If you’re self-employed, you must keep records for six years.
  • Check and check again. Before you hit submit on your tax return, go back through each section to ensure you’ve completed everything you need to. Even something as simple as not ticking a box on a page could result in your return being rejected.
  • In case you’ve made a mistake. You can make changes up to the Self Assessment deadline the following year. So, if you submitted your tax return by 31st January 2022, you can make changes to your 2020/2021 return up to the 31st January 2023.

Paying your Self Assessment tax bill

Once you’ve filed your tax return, you’ll get a bill from HMRC if you owe any tax. It can take up to 72 hours after you’ve submitted your return for the final tax calculation to show up in your account.

If you’re self-employed, you may need evidence of your earnings for various purposes, such as applying for a mortgage. If so, you can request your SA302 tax calculation which shows earnings over the past four years, as well as an overview for a specific tax year. 

If you owe tax, HMRC will tell you the amount you need to pay. You’ll need to pay this by 31st January (another reason why it’s worth submitting your tax return as soon as possible).

Your tax bill can be paid by:

  • Bank transfer (such as CHAPS or Faster Payments)
  • Debit card or corporate credit card
  • Cheque
  • Direct Debit
  • Paying-in slip from HMRC (if you still get paper statements)

Note: You cannot pay your tax bill by personal credit card.

Paying your tax bill in advance with a budget payment plan

If you’re up to date with Self Assessment payments, you can set up a Direct Debit Budget Payment Plan from your online account to make regular payments towards your tax bill. You can choose how much to pay each week or month, but if the total you’ve paid doesn’t fully cover your bill you’ll need to pay the difference by the deadline.

Payments on account

Payments on account are advance payments towards your tax bill. You’ll need to make two every year (by 31st January and 31st July), each one covering half of your tax bill, unless:

  • Your last tax bill was less than £1,000
  • You’ve already paid more than 80% of the tax you owe for the previous tax year

HMRC gives the following example for how this works:

“Your bill for the 2020 to 2021 tax year is £3,000. You made 2 payments of £900 each (£1,800 in total) on account towards this bill in 2020.

The total tax to pay by midnight on 31 January 2022 is £2,700. This includes:

  • your ‘balancing payment’ of £1,200 for the 2020 to 2021 tax year (£3,000 minus £1,800)
  • the first payment on account of £1,500 (half your 2020 to 2021 tax bill) towards your 2021 to 2022 tax bill

You then make a second payment on account of £1,500 on 31 July 2022.

If your tax bill for the 2021 to 2022 tax year is more than £3,000 (the total of your 2 payments on account), you’ll need to make a ‘balancing payment’ by 31 January 2023.”

What happens if I can’t afford my tax bill?

If you don’t have the money to pay your tax bill, you may be given more time to pay or the option to pay in instalments. But you’ll need to contact HMRC as soon as possible to make them aware of your situation.

HMRC will look at your income and expenditure, assets and what you’re doing to get your tax payments in order. They’ll also ask how much you can pay immediately and how long it will take for you to pay the rest.

If you can’t pay before the deadline, get in touch with the Payment Support Service on 0300 200 3835.

Budgeting for your Self Assessment Tax Bill – HMRCgovuk on YouTube

💡 Expert insights

Darren Fell is the CEO and Founder of Crunch, an award-winning online accounting service that supports freelancers, contractors, and practically anyone who’s self-employed. 

For over ten years, Crunch has combined easy-to-use, online accounting software with actual human beings, so that you’re always able to access your accounts and seek the support you need. 

Crunch helps thousands of clients with their accounting needs every single year, using their years of expertise and insight to help small business owners manage their responsibilities.

Q1: What are some processes business owners can adopt throughout the entire year to make Self Assessment easier for them?

Filing a Self Assessment is a long process, and a fiddly one if you’re disorganised or new to the experience, so getting your return organised and submitted early is the best way to simplify the process.

Self Assessment submissions require you to have all kinds of paperwork to hand, such as any P45s, P60s, bank statements, and an organised record of all your invoices and expenses. The key is to keep on top of your bookkeeping and digital records: carve out some time each day or week, depending on which works best for you, to update your records. The more studiously you maintain them, the easier your Self Assessment becomes. Online accounting or bookkeeping software (like we have at Crunch) will let you simplify the process, with cloud software and handy apps to help keep things organised and secure.

Of course, the easiest way to file your Self Assessment is to hire an accountant and get them to do it for you! Even then, the ultimate responsibility for maintaining your records and recording all of your allowable expenses will fall on your shoulders, so start early, and stay organised!

Q2: What are the biggest mistakes business owners make when getting their Self Assessment in order?

Missing the opportunity to claim all of your allowable business expenses is one of the biggest mistakes we see here at Crunch. The reason so many go unclaimed, of course, is simply because it’s not always clear what qualifies: lunches, train tickets, uniforms, office equipment, even your glasses under certain circumstances – they can all be claimed as a business expense, and reduce the tax you pay through your Self Assessment.

Leaving your Self Assessment submission to the midnight hour is another common mistake we see every year. In fact, in 2020, 700,000 people left it to the 31st January deadline – with a nail-biting 26,562 leaving it to the last hour!

Sitting on hold, listening to HMRC’s ear-grating hold music, desperately trying to get in touch with someone before the deadline passes is no way to start a new calendar year. Leaving it late makes it even more difficult to manage any late hiccups or locate any missing expenses, too, which all could’ve been avoided if you’d started the process a few months earlier.

Of course, a lack of organisation is another big problem for a lot of Self Assessment submitters: misplacing vital records or losing your receipts only leads to more headaches down the road, so find some time to update your records each week and back everything up digitally.

File your tax return stress-free

Completing your Self Assessment ensures that you’re paying the right amount of tax, based on what you’ve earned, minus the various expenses you’ve accrued from running your business. In some cases, you may even be owed money from HMRC.

Remember to file your tax return as soon as possible. Take your time to gather the relevant information and complete the form step by step. If you get stuck, refer back to this guide. You can also get help from HMRC’s Self Assessment team online and over the phone, if you need it. 

To stay on top of your finances from the outset, sign up for a free business bank account today. Our handy features, such as accounting software integration and automated expense management, will make keeping track of all things tax a breeze.

Photo by Jopwell, published on Pexels

Adelaide Carleton

Adelaide Carleton

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