How to fill in your Self Assessment tax return

Our tax return guide is for those who are claiming expenses because of their job and not for the self employed.

Knowing how to fill in your tax return and when to submit it is important. Filing your tax return late or incorrectly can be costly, so checking what you need to do before you send in your return is a good idea.

What is Self Assessment?

It’s a process of calculating and paying your income tax to HMRC (also known as the tax office). You don’t have to be self employed to complete a tax return.

The tax office may need you to fill in a tax return even though you are working under PAYE. Self Assessment tax return forms are issued in April every year and cover the previous twelve months.

Getting Started and how to fill in a tax return

It is important to have all of the paperwork to hand when you sit down to begin your Self Assessment. It can be a frustrating experience, especially if you realise that you have mislaid just one important P45 or P60.

It is also a legal requirement to keep your paperwork up to date. Everyone is required to keep their records up to 22 months after the end of a financial year.

The self-employed should keep accounting records for six years from the end of the financial year.

Here’s a basic list of what you will need:

  • P60
  • Details of pay, taxable expenses and benefits received from an employer
  • Statements from a Bank and / or Building Society
  • Cheque and Paying-in book stubs
  • Dividend Vouchers
  • Self-Employment Accounts
  • Documentation regarding Capital Gains
  • Information on other incomes (investments, savings and pensions, property or benefits)
  • Paperwork for anything you might claim in expenses

You won’t typically be expected to send any documents but it will be necessary to keep copies if HMRC subsequently asks to examine your records. If they request this, send photocopies and not the original.

What will I need to fill in my tax return?

The self-assessment pack will include the SA100 form which is the primary document for the Tax Return. You might also receive a couple of other documents depending on whether you are self-employed, earn from Capital Gains or rental income.

Fill in only those that apply – you will not be required to return the rest. HMRC though will usually send you only the forms that are relevant along with a guide on correct completion. Any missing documents can be downloaded at their website.

If your details change, it is important to let them know as soon as possible. Fines and penalties apply only to carelessness and deliberate attempts to cheat the system.

How can I fill in my tax return?

Tax Return Online

This is now the most popular and convenient way to complete your tax return. If you’re not already logged on you’ll need to register online for the service.

Filling in a tax return online eliminates the potential for your forms getting lost in the post – saving a lot of worry for you and hassle for HMRC.

Online submission also gives you three months to pay, you can save your progress as you go so you do not have to do it all in one sitting.

Once completed, you receive an immediate acknowledgement instead of anxiously waiting for the confirmation of processing.

The software system of HMRC will calculate what you owe in tax and present you with a figure. If you are owed a tax rebate, you should receive this quicker.

It can take up to a week to receive to receive your account password and be fully registered on the site so register as early as possible so that you can be sent an activation pin from the website.

You will need your UTR (Unique Taxpayer Reference) which be on your SA100 form, your National Insurance Number and / or your postcode.

Tax Return by Post

The deadline for postal mail is 31st October every year. Allow plenty of time if you choose to send via this method.

Ensure that all documents are filled out fully and correctly. Even something as simple as all relevant signatures and dates are vital in the process.

It cannot be stated enough that you need to take a photocopy in case it gets lost in the post. HMRC will not notify you of receipt, but they will send you notification when your tax return has been processed.

If you can get your paper returns to HMRC by the 31st October deadline, they will calculate it for you.

They will also inform you of what you owe. If the sum is under £2000, they may be able to collect it through your tax code rather than as a lump sum in January.

What are the Self Assessment deadlines?

The tax year starts on 6th April every year and ends 5th April the following year. You’ll receive your tax return papers every year and the deadlines will vary depending on how you plan to go about filing your tax return:

  • 31st October: If you plan to fill your return on paper.
  • 31st January: If you plan to complete your record online.
  • 31st January: Any tax owing must be paid by this date though some will have to make additional payments in July.

If you miss the deadline, even by a single day, you might face a penalty of £100 regardless of whether you owe no tax or whether you have paid. If you do not settle immediately, you will incur extra charges and interest. Further charges will apply if you are significantly late.

Start your tax return as early as possible and you can avoid all of these problems and have time to make amendments if needed.

You’ve completed your tax return

Assuming everything goes to plan, you will receive your acknowledgement and how much you owe / are owed in next to no time.

For some people, there will undoubtedly be frustrations. In some cases you might need the assistance of a professional accountant to make sense of the forms, the process and how to work it all out.

How to complete your tax return useful tips

  • Sign the Tax Return on the final page (if you’re completing it on paper).
  • If you want to complete your tax return online by the end of January, you must register on the website.
  • Only fill in applicable boxes. Writing ‘nil’, ‘0’, ‘n/a’ and ‘not applicable’ is unnecessary.
  • It isn’t necessary to send paperwork, but you must keep copies for two years after the end of the tax year. In some cases, (lettings and other property) you’ll need to keep records for six years.
  • Claim any tax relief you might be entitled to for example job expenses related to your job.
  • Don’t worry if you cannot give exact amounts as you are permitted to make estimations. If your expenditure on something like stationery is around £10 per month and you only have half the receipts, you are permitted to project the half-year expense for the whole year.
  • If you are waiting for further information that might affect your assessment, you can put it in provisionally with a clear note stating that this is only the latest information – final details to follow.
  • The final two pages are for any notes only. This is to explain any issues you feel need highlighting. This would be the ideal place to explain any discrepancies.

Making Tax Digital and Income Tax for Self Assessment

Making Tax Digital for ITSA is gradually bringing in a different process for people who have business or rental income to submit their tax information to HMRC.

Sole traders and landlords with a certain income will need to keep digital accounting records and submit income and expenses quarterly instead of an annual tax return.

From April 2026 if you have business or rental income of more than £50,000 you will need to comply with MTD for ITSA regulations.

The MTD for ITSA process has a number of different features like quarterly reporting, an end of period statement (EOPS) and final declaration.

Submitting your tax information through MTD for ITSA replaces the need for an annual tax return.

To learn more about how MTD for ITSA could effect your business you can read our MTD for ITSA guide.


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