Income tax for Self Assessment: The complete guide for accountants

As an accountant, keeping up with changes to tax regulations is a key part of the job. The government has been rolling out its Making Tax Digital (MTD) initiative over the last few years, and they frequently update key information relating to the scheme. 

So what do you need to know? 

From April 2026, Making Tax Digital for Income Tax (ITSA) will come into force for those who earn qualifying income over £50,000. Sole traders and landlords who file Self Assessment returns and meets the threshold will need to keep digital records and submit quarterly updates on their finances to HMRC.

In this article, we’ll run through some of the ITSA basics and give you some Self Assessment for accountants top tips to help you make sure your clients are ready. We’ll also show you how our accounting software for MTD can make the whole process simpler for you and your client. 

What is Self Assessment Income Tax?

Self Assessment Income Tax is a system where individuals calculate and report their own tax liability to HMRC. It applies to individuals with income not taxed at source, such as freelancers or those with rental income, requiring them to file an annual tax return and pay the correct amount of tax directly.

The Self Assessment Income Tax process is simple: 

• - You report how much you earned over the last tax year and subtract any expenses

• - HMRC calculates how much tax you owe

• - You pay HMRC by your preferred method (direct debit, bank transfer or cheque, for instance)

People with smaller or simpler businesses often submit their return and pay what they owe themselves. But those with larger or more complicated accounts tend to employ professional accountants to take care of the process for them. 

Who needs to file a Self Assessment Income Tax return?

The main set of people who need to file a Self Assessment process for income tax are self-employed sole traders, landlords and partners in business partnerships. 

All partners must complete a return, but sole traders and landlords only need to do so if their turnover was higher than £1,000 in the last tax year (6 April to 5 April). 

Income outside of these sources must also be reported. This might include: 

• income from savings, investments and dividends

• tips and commissions

• foreign income 

• money from renting out a property 

You can learn more about who is eligible and why on the government’s Self Assessment tax returns page.

Like a lot of the UK tax system, Self Assessment can seem a little complicated, and the rules on who needs to fill out a return aren’t always clear or easy to find. 

What are the MTD requirements for Income Tax?

MTD is a government initiative to make tax digital. As part of this, there are a number of new requirements that accountants need to be aware of for Self Assessment Income Tax. 

Under the new rules, eligible businesses will need to do the following: 

• Use software for initial accounting, or transfer their accounts into electronic data. 

• Submit this electronic data to HMRC in an HMRC-approved format. 

• Make quarterly reports in addition to annual Self Assessment tax returns. 

If you have clients with a VAT-registered business, you’ll already be familiar with the basics of Making Tax Digital, as MTD for VAT was made mandatory in April 2022.

When will the MTD rules apply to Income Tax?

MTD rules will apply to income tax from April 2026. That’s when the government plans to roll out MTD ITSA for businesses and landlords with an annual turnover of more than £50,000. From April 2027, it will also affect businesses and landlords with an annual turnover more than £30,000.

However, it makes sense to be well prepared for MTD so you can get your clients up to speed ahead of the deadline.

What information is required to complete a Self Assessment Income Tax Return?

To complete a Self Assessment Income Tax return, you'll need to gather all of your client’s relevant financial data for the tax year (6 April to 5 April). 

Here's a checklist of the key information required:

Personal information

• Unique Taxpayer Reference (UTR): A 10-digit code assigned by HMRC.

• National Insurance Number.

Income details

You'll need to report all income earned, including:

• Self-employment income

• Income from property

• Employment income: include your P60 or P45 if your client has worked as an employee.

• Dividends and savings

• Foreign income: any income earned abroad that is taxable in the UK.

• Other income sources: this might include tips, commissions, or any other non-salaried income.

Expenses and deductions

Deductions help reduce your taxable income. Gather records for:

• Allowable business expenses: these could include office supplies, travel, marketing, or other operational costs.

• Property-related expenses: such as repairs, property maintenance, and mortgage interest.

• Charitable donations

Tax Reliefs and Allowances

• Pension contributions

• Marriage Allowance

Records for Tax Paid

• Include any tax already paid through PAYE or other means. This helps ensure your client is not taxed twice on the same income.

Relevant Accounting Records

If you're using Making Tax Digital (MTD) software, ensure all of your accounting records are updated electronically and submitted in the required format.

For more information on how to fill in a Self Assessment tax return, explore our comprehensive guide to Self Assessments. 

How to pay Self Assessment Income Tax online

When it comes to paying Self Assessment Income Tax, HMRC provides several options to make the process straightforward. Your clients can pay via direct debit, bank transfer, debit or credit card, or by cheque. Most people choose to pay online, which is quicker and easier. 

There’s a lot of information and advice out there on Self Assessment for accountants looking to streamline their clients’ accounts, but the best way to put your and your clients’ minds at ease is to use QuickBooks. 

With our MTD enabled software, you’ll be able to collect all relevant data to give an accurate and up-to-date view of your financial position. 

Self Assessment Income Tax deadlines

It’s crucial to remember all of the relevant Self Assessment Income Tax deadlines in order to prevent your client being hit with any penalties or fines. 

Under the new system, your first quarterly return under the new system is due in the fourth month of your accounting period. You’ll then need to file another return with HMRC every three months. 

This is a significant change. At the moment, Self Assessment returns are filed annually, but under MTD, businesses will need to submit these quarterly reports as well. 

For an accounting period that matches the tax year, your reports would be due as follows: 

• 1st quarter (6 Apr to 5 Jul) is due 5 August 2026

• 2nd quarter (6 Jul to 5 Oct) is due 5 November 2026

• 3rd quarter (6 oct to 5 Jan) is due 5 February 2027

• 4th quarter (6 jan to 5 Apr) is due 5 May 2027

• 31 January 2026: End of Period statement (EPOS) and final declaration due 

HMRC still hasn’t confirmed final dates for payments, but many expect that there will also be an option to make payments throughout the year.

Note: There won’t be any late filing penalties for at least one year from April 2026, but HMRC has made clear it will charge penalties after the grace period, so it’s crucial to get on top of the new system as soon as possible.

Stay on top of Self Assessment with Quickbooks

Looking to streamline your accounting practices and keep your clients’ compliant? 

If so, QuickBooks Online Accountant is the ultimate tool for you. With an account, you can manage everything from client accounts to payroll and cash flow, all from one central hub*. QuickBooks helps you handle everything from day-to-day accounting to Making Tax Digital (MTD) compliance with ease.

And, with real-time support seven days a week, you'll always have expert help when you need it.

* Applies to certain offers over certain periods. 

We hope this article has helped explain both the basics of filling out a Self Assessment Income Tax return and how the new MTD scheme is set to change it. 

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