Tax Return: a Step-By-Step Guide to Completing Your Self Assessment

Whether you wish to find out how to fill in your tax return for the first time or you’re just looking for a comprehensive checklist to make your approach more efficient, read on for GoSimpleTax’s handy tax return guide.

How to fill in your tax return, step by step

Step One: Do I Need to Complete a Tax Return? Find Out and Register

If you’re not already registered for self-assessment, you can check whether you should be here.

You should register for self-assessment before the 5th October (after end of the tax year). If you miss this deadline you must register at least ten days before your first tax return is due. Your registration takes only a few minutes and should be completed here.

When registering for your self-assessment, you should have the following to hand:

  • Your National Insurance Number
  • Your activation code
  • Basic details of your business (if a sole trader) including name and address.

You’ll receive a letter within ten days (or 20 days if you’re registering from abroad) containing a ten-digit Unique Taxpayer Reference (UTR) number and another containing an activation code (this will expire after 28 days). Though it’s worth noting that if it’s your first time filing, you may not have a UTR number.

Once you have found out whether you need to file, follow this handy self-assessment guide to help you breeze through the process.

Step Two: When Can I File My Tax Return?

Tax years run from 6th April each year to 5th April the following year.

The annual deadline for self-assessment is 31st October for paper tax returns and 31st January for online tax returns.

There are some tax filing mistakes to avoid, such as submitting your self-assessment late. To avoid being fined, you should register by no later than the 5th October.

Penalties for the late filing of tax returns are as follows:

  • Non-refundable £100 fine for missing the 31st January (31st October for paper self-assessment)
  • After three months’ without filing, £10 per day, continuing up to £900.
  • After six months, an extra £300 (or 5% of what you owe, depending on which is greater)
  • After twelve months, an additional £300 (or 5%) – though there is the potential to be fined up to 100% of your owings.

These still penalties apply even if no tax is owed.

If you’ve filed your return but are late in paying, you’ll be fined:

  • 5% of the amount owed after 30 days of non-payment
  • A further 5% after 6 months
  • 5% more after 12 months

You can appeal against some late tax return penalties using an SA370 form, which can be found here.

Step Three: What Information Do I Need?

Being unprepared is one of the major tax filing mistakes to avoid. Here’s everything you’ll need:

  • Full details of your business’s income and expenditure if you are a sole trader. You might be able to claim some expenses back – take a look here for further information.
  • Full details of income earned from employment which may include your P60, P45 if you left during the tax year. Your P11D form, detailing any benefits and expenses you’ve received that weren’t processed through the payroll system, Your P2 coding notice, which explains your PAYE tax code. Along with details of any business mileage reimbursed to you by your employer. If you were made redundant then you will also need details of any redundancy payments received to hand.
  • A total of any rent you have received
  • Certificates detailing interest you’ve received from your bank
  • Any income you’ve received from overseas
  • Any income you’ve received as part of a partnership (one partner should also file a tax return for the partnership as a whole)
  • Information about any dividends received
  • All taxable benefits you’ve received from the state, All capital gains you’ve received by disposing of assets
  • Information about any Gift Aid you have received (you can claim tax back for this – see here for details)
  • Details of any pension contributions, as you may be able to claim some of this money back – find out more here
  • Details of any tax payments you’ve already made this year (known as payments on account – more on this here)

The above information should only refer to the previous tax year – so the 6th April of the previous year to the 5th April of the current year.

Step Four: Take Your Time

Now you can fill out your tax return slowly and carefully. You can always save it and come back to it if you’re doing it online, so there’s no need to rush. Alternatively, filing your self-assessment with GoSimpleTax is simple and easy for those of you who dread sorting out your tax affairs. 

You should check back through all the information you’ve entered before you press “Submit” – you can edit everything at any point before this, but realising you’ve filled out your tax return wrong afterwards can lead to implications.

Step Five: Make Your Payment

The amount you owe will be calculated immediately after you submit your tax return. It’s vital that you pay it before the deadline.

If you think you may be forced to pay it late, you can notify HMRC’s Business Payment Support Service via 0300 200 3835.

You can also set up a budget payment plan here to submit amounts in advance.

To help you correctly file your tax return online with no unnecessary stress, it’s worth investing in helpful resources such as GoSimpleTax’s mobile app or online software, which will allow you to upload receipts in real-time and automatically calculate your tax for you, immediately alerting you of any potential mistakes or opportunities to claim amounts back.

This software will also help you quickly and easily submit all information direct to HMRC.

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