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How to Complete a Self Assessment Tax Return

Have you never done a tax return before and are about to fill in your first one? Maybe you’ve done tax returns before but want to brush up on the basics. Either way, this guide will help you fill in your next Self Assessment tax return and answer any questions you may have.

What is a tax return?


A Self Assessment tax return is used to calculate how much tax you need to pay on your income. Usually, tax is paid via PAYE (pay-as-you-earn) and your wages are taxed before they reach your bank account.

However, if you’re self-employed, then you’ll have to complete a Self Assessment tax return to work out how much tax you need to pay on your income. 

It’s your responsibility to inform HMRC that you need to fill in a tax return and file for a Self Assessment. If you’re a full-time employee that receives payslips and that’s your only income, then you won’t need to complete a tax return. However, if you’re earning other income under self-employment, then you will need to report this. 

We explain who needs to complete a tax return below. 

Do I need to do a tax return?


Most people have tax automatically deducted from their wages and pensions and so don’t have to worry about filing a tax return each year.

However, if you don’t have tax deducted from your wages because you're self-employed, for example, then you will have to do a Self Assessment tax return.

If any of the following applies to you, you’ll need to submit a self-assessment tax return:

  • Your self-employed income was more than £1,000.
  • Your income from rented properties was more than £2,500.
  • You earned more than £2,500 untaxed income. For example, from tips or sales commission.
  • Your income from savings and investments was £10,000 or more.
  • You’re a directory of a company.
  • Your income was more than £50,000 and you or your partner claimed child benefit.
  • You’ve earned income abroad that you need to pay tax on. 
  • Your taxable income was over £100,000.
  • You’re a trustee of a trust or registered pension scheme.
  • Your state pension was more than your personal allowance (£12,570) and was your only income.
  • You received a P800 from HMRC saying you did not pay enough tax last year.

If you’re still not 100% sure whether you need to file a tax return, you can use the Government’s online checker.

Self Assessment tax return deadlines

Here are the current tax return deadlines for the 2020/21 tax year which ran from 6th April 2020 to 5th April 2021:

Register for Self Assessment5th October 2021
Submit a paper tax return (if you prefer to online)Midnight 31st October 2021
Submit an online tax returnMidnight 31st January 2022
Pay any tax you oweMidnight 31st January 2022

You also make a second tax payment on the 31st July if you make advance payments on your tax bill. 

What if I missed the tax return deadline?

If you miss any of the deadlines mentioned above, then you’ll have to pay a penalty unless you have a reasonable excuse

The penalties for failing to submit a tax return by 31st January are as follows:

  • £100 penalty if you fail to submit your tax return by 31st January.
  • £10 penalty a day for up to three months after your tax return was due. A maximum of £900.
  • After six months, you’ll be charged another penalty of 5% of the tax owed or £300.

There are also penalties for paying your tax late. You’ll be charged 5% of the unpaid tax after 30 days, six months and 12 months.

If you’re worried, the Government provides an online tool to calculate how much you’ll pay in penalties if you miss the tax return deadline.

How to register for a Self Assessment tax return


If you’re submitting your tax return for the first time, you will first need to register for Self Assessment. Here are the steps you’ll have to take:

  1. Register with HMRC - you can register for Self Assessment online
  2. Receive your Unique Taxpayer Reference (UTR) number - HMRC will send your UTR number in the post once you’ve registered. This letter instructs you on how to set up your Government Gateway account.
  3. Use your activation code to set up your Government Gateway account - once you’ve gone through the steps outlined in the letter, you’ll be sent another letter containing an activation code. This code is needed to activate your account so use it quickly as it expires!
  4. Complete your account set-up - make sure your Government Gateway account is successfully activated as only once this is up and running will you be able to submit your tax return online.

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


Filing your Self Assessment tax return for the first time can seem intimidating. However, if you have all the information you need to hand, filing your tax return is a straightforward process.

Here’s a list of the information you need to complete a Self Assessment tax return:

  • Your Unique Taxpayer Reference (UTR) number.
  • Your National Insurance Number.
  • Information on your total untaxed income from the tax year. This includes income from self-employment and interest on investments etc.
  • Records of your expenses related to your self-employment (office running costs etc.)
  • Contributions to charity or pensions which might be eligible for tax relief.
  • P60 showing how much income you’ve already paid tax on.

We recommend that you download the tax return help sheets on the Government website to gain a better understanding of the information required.

How to fill in a Self Assessment tax return


You can submit your tax return either online or via a paper form. Most people complete their Self Assessment online because the deadline is later and you can submit it right away and review any mistakes. 

We will give a summary of how to fill in an online and paper Self Assessment tax return below:

How to fill in an online tax return


Once you’ve logged into your Government Gateway account, you’ll need to follow these steps to complete your tax return:

Step 1: Have all your information ready

Make sure you have your P60, bank statements, invoices and receipts to hand to help fill in your income and expenses.

Step 2: Check your personal details are correct

If any information is inaccurate, you could face a penalty. Make sure your personal details are correct by checking your Government Gateway account.

Step 3: Fill in the sections that apply to you

With online tax returns, you only need to fill in the sections that apply to you and your situation. So, if you aren’t a business partner, you can skip that section, for example.

The main information you’ll have to provide includes the following:

  • Income - all taxed and untaxed income throughout the tax year.
  • State pension - the state pension payments you received (if relevant.)
  • Benefits - any benefits you might have received.
  • Pension contributions - pension contributions made after tax.
  • Charitable donations - total amount of donations you’ve made to charity.
  • Blind Person’s Allowance - confirm whether you are receiving this or not.
  • Student Loan repayments - total any Student Loan repayments made.
  • High-income child benefits - for those receiving child benefit when they, or their partner, earn more than £50,000.
  • Marriage allowance - the marriage allowance means you can transfer some of your personal allowance to your partner.

Step 4: Check your tax return

Check to make sure that the information you have provided is correct. With online tax returns, you can amend your tax return before submitting it.

Step 5: Submit your tax return

Once you officially submit your tax return, you’ll receive a confirmation and code number on screen. Make a note of this to make sure you can reference it for any future tax enquiries.

Step 6: Pay any tax you owe

Once you’ve submitted your tax return, HMRC will work out how much tax you owe and you’ll be able to see the figure on your online Government Gateway Account. You have to pay tax by 31st January. We explain more about this later.

How to fill in a paper tax return


As of April 2020, HMRC no longer automatically sends out paper tax returns. If you’ve filed a paper tax return in the past, you’ll receive a reminder but HMRC will encourage you to fill it in online.

If HMRC has identified that you can’t file online, they will send you a paper tax return. Paper tax returns must be completed by 31st October each year, much earlier than online tax returns.

Sections of a paper tax return

There are lots of different sections on a paper tax return which can be overwhelming. Here’s a summary of each page:

  • Page 1 - Personal details such as date of birth and phone numbers.
  • Page 2 - Summary of types of income you receive throughout the year - tick all that apply to you.
  • Page 3 - List other sources of income such as savings and investments.
  • Page 4-5 - Common types of tax relief such as marriage allowance, Gift Aid and child benefit.
  • Page 6 - Record any previously refunded tax. Provide a bank account for any possible tax refunds.
  • Page 7 - Tax advisor details, if you have one.
  • Page 8 - Sign your tax return and explain if any figures are provisional.

Supplementary pages

You might need to fill out supplementary pages under certain circumstances. For example, you made capital gains.

HMRC might send out these pages automatically. If they don’t, you can download the relevant ones here.

Paying your Self Assessment tax bill


The deadline for paying your Self Assessment tax bill is the 31st January. You’ll need to have paid the following by this date:

  • Any tax owed for the previous tax year.
  • The first half of your tax for the following year.

For example, if you paid £2,000 in tax for the previous tax year, you’ll have to pay £1,000 in advance for the first half of the following year.

This advance payment helps to spread your taxes out and reduces the size of the total lump sum due. 

How can I pay my Self Assessment tax bill?

You can pay your tax bill by bank transfer, debit card or cheque. You can also pay at your bank if you have a paying-in slip from HMRC.

Payments to HMRC can go in as fast as two hours but we don’t recommend paying your tax bill last minute! 

Try to pay your Self Assessment tax bill at least a few days before 31st January. 

What if I can’t afford to pay my tax bill?

If you can’t afford your tax bill, contact HMRC right away to avoid late payment penalties and agree to spread your payment over a period of time. 

You will need a reasonable excuse to not pay your tax bill on time. This can include:

  • Your partner or close relative died just before the tax return deadline.
  • You had an unexpected hospital stay.
  • You had a serious/life-threatening illness.
  • Your computer failed right before you were due to send your tax return.
  • You had issues with HMRC’s online services.
  • A fire, flood or theft prevented you from completing your tax return. 


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