What is Self-Assessment?

Your Self-Assessment is a standard tax return form. As a business owner, you’ll need to send a report of your annual earnings to HMRC. Your Self-Assessment tax return should also include the sources of your earnings.

It’s called Self-Assessment because it is your responsibility to work out how much tax you have to pay.

When do I file my Self-Assessment tax return?

If you’re going to file your self-assessment tax return online, this will need to be done by 31 January after the end of the tax year.

If you’re not filing your return online, it needs to be done by 31 October after the end of the tax year.

The tax year runs from 6 April to 5 April of the following year.

FAQ: What if my accounting year is different to my tax year?
Answer: if this is the case, you report the earnings for the accounting year that finished during the tax year.

Where do I submit my Self-Assessment tax return?

The easiest way to submit your tax reform is to use the online self-assessment HMRC login.

Remember to file your tax return after it has been filed. You need to keep records for up to 6 years prior. HMRC can issue a fine for each tax year you don’t have the correct records for.

What do you need to include in your Self-Assessment tax return?

A Self-Assessment tax return will include, amongst other things:

  • Your P60 form, which is a summary of your income and tax deductions
  • Your P11D form, which covers expenses and benefits
  • Details of other personal income and investments
  • Capital gains from the sale of assets, including shares and property
  • Any taxable benefits from employers or the government
  • Your National Insurance (NI) number and employer reference, if you have one

If you’re filing online, HMRC will provide other information like state pension, underpayments and some NI contributions.

Some tax breaks will be included in your tax returns. You won’t, however, need to include any tax-free income earned.

Preparing for your Self-Assessment tax return

HMRC can charge penalties if our records aren’t complete and accurate so it’s important to keep your books properly.

Try and get copies of bank statements, invoices and other important paperwork to support your claims. If you can’t get these copies, offer “provisional” or “estimated” figures. Just make sure you say this is what you’re doing in the “Any other information” box on the tax form.

One of the most effective ways to prepare your books for your Self-Assessment tax return is to use accounting software.

Self-Assessment software has a number of advantages. Primarily, it allows you to upload invoices and receipts each month, which makes it easier for you to go through your year’s financial activity. This in turn helps make your Self-Assessment easier and more accurate.

Do I need to sign up for Self-Assessment?

You will need to submit a Self-Assessment tax return if you’re self-employed, or have taxable gains to declare to HMRC (for example, you are a landlord). The UK GOV website has a full list of who must send a tax return.

You’ll also need to file the SA tax return if you’re the director of a company or earn over £100,000 per annum.

If you pay income tax through a Pay As You Earn (PAYE) scheme, then you won’t need to self-assess.

Signing up for Self-Assessment

If you’re filing online for the first time, make sure you have your Unique Taxpayer Reference (UTR). You can find this on your HMRC registration letter. Then enrol for the online service, and activate it using the code HMRC send you in the post.

You can sign up for Self-Assessment here: https://www.gov.uk/log-in-file-self-assessment-tax-return

Self-Assessment in KashFlow

In KashFlow, there’s a Self-Assessment report that gives you the figures you’ll need to complete your Self-Assessment form.

These are created by mapping your Sales and Outgoing Types Codes to the correct fields on the Self-Assessment form. This means you don’t need to enter data twice. We do, however, advise that you check the figures with an accountant before you submit them to HMRC.

Want to get ahead with your Self-Assessment tax returns? Just create an account with KashFlow and get going in a couple of clicks!


Creating a Self-Assessment report in KashFlow’s Accounting software can be done in a couple of clicks.

Set where Sales, Outgoing and Bank Transaction Type Codes are classified on the Self-Assessment report by following these steps:

  • Sales > Sales Codes > Click on a Sales Type Code > Change the Self-Assessment drop down as appropriate > Update
  • Purchases > Purchase Codes > Click on an Outgoing Type Code Change the Self-Assessment drop down as appropriate > Update
  • Bank > Bank Codes > Click on a Bank Transaction Type Code > Change the Self-Assessment drop down as appropriate > Update

Support cannot advise on how to classify any Nominal Code. If you are unsure you should speak to your accountant.

Running the Self-Assessment Report:

To run the report, go to:

  • Tools & Reports > General Reports > Self Assessment

Use the “Start Date” and “End Date” fields to specify a date range for the report and click “Re-run Report”. Your Self-Assessment Report will then be generated field-by-field and display towards the bottom of the page, along with the corresponding value.

To download the figures in a CSV format, click on the download button next to the field value. Similarly, you can print this report by clicking the print button.

See how KashFlow works with your business and your books