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Self Assessment tax returns – Everything you need to know

Self Assessment tax returns header image

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

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 and you alone. 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 intended for certain people whose income isn’t taxed automatically when it’s paid. We’ll discuss the criteria for being required to file a Self Assessment in the next section.

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 a paper form or online 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.

Quick 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 familiarize yourself with how an accountant can help. To learn more, read our complete guide to accounting for startups.


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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:

Note: *The tax year runs from 6 April until 5 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.

  • Earned more than £1,000 as a self-employed sole trader
  • Were a partner in a business partnership
  • Were a company director (unless you acted as a director for a non-profit organisation and received no benefits, such as a company car or travel expenses)
  • Earned more than £2,500 from untaxed income (for example, if you earned commission through work)
  • Earned more than £2,500 from renting out property
  • Earned more than £100,000 in taxable income
  • Earned £10,000 or more before tax from savings and investments
  • Have income from overseas that you need to pay tax on
  • Lived abroad but have income from the UK that you need to pay tax on

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 that was more than your personal allowance and was your sole income
  • Earned over the threshold (£46,351 for 2018/19; £50,001 for 2019/20) and made pension contributions. In this case, Self Assessment can be used as a way to claim tax relief.

My first Self Assessment tax return – HMRCgovuk on YouTube

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 on a used pair of shoes that you were fond of, this points to the sale not being 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. HMRC has different methods for registering depending on your employment status. Here are the links for each:

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 2019/20 tax year, you’ll need to register by 5th October 2020. If you miss this deadline, 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.

If you’ve registered in the past, you won’t need to register again. But you will need your old UTR to set up the account.

Quick 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 and your tax bill is less than £3,000, HMRC can automatically collect what you owe from your wages and pension if you:

  • Submit your paper tax return by 31st October or your online tax return by 30th December after the end of the tax year.

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 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 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 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

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

  • A person you were relying on to complete your tax return didn’t do it
  • 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

How to complete your Self Assessment tax return

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Easy finance admin on web and mobile with Tide (source: #wocintechchat, edit Tide)


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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 income from self-employment including bank statements, dividends and interests on shares
  • Details of any self-employment business expenses
  • Details of any pension or charitable contributions that may to eligible for tax relief

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

Quick tip: It’s important to organise your finances as soon as possible after you launch your small business. The sooner you methodologically track your income, expenses, invoices, and so on, the easier things will be when it comes time to filing taxes. To learn more, read about how Tide’s expense management feature can make your tax life easier.

Filling in the Self Assessment tax return

As long as you have accurate records of your income and expenses, filling in the tax return 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.

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:

  • 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.

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 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

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.  

For UK property

Your income is the total amount you’ve earned from any furnished holiday lettings and from any other properties you rent out.

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 expenses on GOV.UK.

For capital gains

Your income is your total disposal proceeds for residential and non-residential property, shares and securities.

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

What expenses can I claim on capital gains?

  • Stamp duty paid when buying the property
  • Buying and selling costs such as estate agent fees, solicitor fees and surveyor fees
  • Improvement costs such as a new kitchen or bathroom or decorating work

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

Once you’ve entered all of the information, hit Submit and you’re all done. HMRC will then calculate if you owe tax and how much, as well as the 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 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 either. 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 2020, you can make changes to your 2018/19 return up to the 31st January 2021.

Paying a Self Assessment tax bill

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
  • Cheque
  • Direct Debit
  • Paying-in slip from HMRC

You can’t pay your tax bill by 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

HMRC gives the following example for how this works:

Your bill for the 2017 to 2018 tax year is £3,000. You made 2 payments on account last year of £900 each (£1,800 in total).

The total tax to pay by midnight on 31 January 2019 is £2,700. This includes:

  • Your ‘balancing payment’ of £1,200 for the 2017 to 2018 tax year (£3,000 minus £1,800)
  • The first payment on account of £1,500 (half your 2017 to 2018 tax bill) towards your 2018 to 2019 tax bill
  • You have to pay your second payment on account of £1,500 by midnight on 31 July 2019.

If your tax bill for the 2018 to 2019 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 2020.

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 (PPS) on 0300 200 3835.

Budgeting for your Self Assessment Tax Bill – HMRCgovuk on YouTube

Wrapping up

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 as soon as possible and to take your time. 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 Tide business current account today. Our handy features such as accounting software integration and automated expense management will make keeping track of all things tax a breeze.


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Photo by Jopwell, published on Pexels

Shahree Zin

Partnerships Executive and small business accounting advocate

Tide Team

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