How to Register for Self-Assessment for the 2025-26 Tax Year

If you’ve earned over £1,000 as a sole trader during the 2025-2026 tax year and have yet to pay tax on it, you need to register for HMRC Self-Assessment.

This also applies if you’re an employee but have additional revenue streams you still need to pay tax on. This could include income from property rentals, selling sites, or creative side-projects that have taken off this year. You also have to file an HMRC Self-Assessment tax return if you:

• Are a company director or partner
• Have income from savings such as pensions, investments, and dividends
• Have foreign income
• Pay the High-Income Child Benefit Charge

Registering for HMRC Self-Assessment

The first step to getting your tax payments in order is to register yourself with HM Revenue and Customs (HMRC). This is a straightforward process that can be completed online or via post using a SA1 form, which can be downloaded from the government website.

Although postal applications are currently still open, paper-based returns are being slowly phased out as Making Tax Digital (MTD) comes into effect as of April 2026. Initially this will only affect sole traders and landlords earning over £50,000, but eventually paper-based tax returns will become a thing of the past! (more on this later).

If you find you need to register for Self-Assessment for the 2005-26 tax year, the deadline for doing this is 5^th October 2025.

How to create a self-assessment HMRC account

The registration process is quick and easy; it only takes around 10 minutes to do it online, following the step-by-step instructions. You’ll also be able set up your Government Gateway account at the same time.

This online government portal lets you to pay your Self-Assessment tax return and National Insurance Contributions to HMRC digitally using your Unique Taxpayer Reference (UTR) and a personal gateway password.

You can also use the Government Gateway to make advanced payments on your tax bill throughout the year. This is known as ‘payments on account’.

Once you’ve registered for Self-Assessment online, you’ll receive your UTR by post within 14 days. You can get it sooner by using the HMRC app.

Missed the deadline to register?

If you miss the registration deadline on 5th October, you should still complete your Self-Assessment registration as soon as possible.

Provided any Income Tax or National Insurance Contributions due are paid by the 31^st  January 2026, there will be no loss of tax revenue. This means you won’t be liable for a ‘failure to notify penalty’, which is calculated by HMRC as a percentage of the tax revenue lost.

It can take up to four weeks for HMRC to process your registration and issue your Unique Taxpayer Reference (UTR). For this reason, it is advisable to submit your application sooner rather than leaving it until the last minute. Taking the time to do it now will help you stay organised and avoid unnecessary stress later on.

Penalties for late tax payments

Penalties for missing payment of your Income Tax Return are more severe and should be avoided wherever possible.

Missing the tax filing deadline incurs a £100 fine; if it’s more than three months late, you could be charged an additional £10 a day up to the value of £900.

In addition to this, late payment of Income Tax is charged at:

5% of the tax unpaid after 30 days

Another 5% of the tax unpaid after 6 months

Another 5% of the tax unpaid after 12 months

Even if you only owe a small amount in tax, these late fees can add up fast. If you are in any doubt about how much tax you owe or if you need to pay Income Tax at all, speak to your accountant.

Personalised Self-Assessment help and support

Another way to avoid the stresses of registering for Self-Assessment and completing your tax return in good time is to let CTT Accountancy take care of it for you.

We specialise in providing our clients with a one-to-one service, getting to know you and your business to understand your unique tax needs.

Whatever level of support you require with your Self-Assessment, whether it’s casting a trained eye over your submission before you press ‘send’ or completing the whole process for you from start to finish, our expert team is on hand to help answer your questions and deliver stress-free compliance with HMRC regulations.

Preparing to submit your tax return

To submit your return, you’ll need to follow information and documents if relevant to you:

• Your 10-digit Unique Taxpayer Reference (UTR)
• Your National Insurance (NI)number
• A P60 from your employer. This should show your income and any tax you’ve already paid
• A P45. This is only necessary if you’ve left a job during the current tax year
• A P11D or P9D. This should show any benefits you receive and expenses
• A summary of any income you have from rentals and expenses incurred
• Statements of investments and savings that show how much interest you’ve earned and any other revenue streams such as dividends
• Any receipts, bank statements and accounts detailing your self-employment income
• Evidence of any contributions you’ve made to charities or pensions that might qualify for tax relief

Submitting your Income Tax Return

There are two ways to submit your Income Tax Return – via post and online. To submit your return by post, you’ll need to download an SA100 form from the government website.

Once you’ve completed your return form, send it to HMRC by post before the 31^st October 2025.

A faster, more convenient way to submit your return is to do it online using your Government Gateway password and UTR, the deadline for electronic filing is 31^st January 2026. The government’s Making Tax Digital (MTD) initiative comes into force in April 2026 for sole traders and landlords earning over £50,000. Making the move now to submit your return digitally will stand you in good stead as eventually MTD will be the only way for anyone to submit a return to HMRC.

Digital Accounts and Tax Returns

Digitising your accounts doesn’t just save you time and reduce stress when it comes to tax returns – it also minimises the risk of errors or omissions and highlights trends that could show opportunities for potential tax savings.

At CTT Accountancy, we make the transition to digital simple with a tailored onboarding process that helps smooth the move from paper-based processes, including showing you how to register for self-assessment.

We also offer exclusive discounts on leading bookkeeping software, including Sage, Xero, and QuickBooks, along with expert guidance to help you choose the platform best suited to you and your business.

When it comes to filing your HMRC Self-Assessment online, we’re here to help you every step of the way. See our full list of services and contact us today.