A growing number of people across the UK are generating income in new ways, from reselling items online and running side businesses alongside full-time employment to becoming self-employed and letting out property.

However, leading South West accountancy firm Milsted Langdon is warning that many people are unaware they need to register for Self Assessment until it is too late, and fears that some may miss the upcoming registration deadline.

Anyone who became liable for Self Assessment during the 2025/26 tax year must register with HM Revenue & Customs (HMRC) by 5 October 2026.

Zoe Chandler, Senior Tax Manager at Milsted Langdon, said: “The requirement applies to anyone who started self-employment or began letting out property at any point between 6 April 2025 and 5 April 2026, as well as anyone who received other income that was not taxed at source, such as earnings from a side business or significant income from online selling above the £1,000 trading allowance.

“Every year, people are caught out by this and can later find themselves facing unnecessary issues with HMRC. If anyone is unsure about their position, they should seek advice rather than risk missing the deadline.”

While registering late will not prevent someone from filing a tax return in January, those who fail to notify HMRC of their liability on time may face penalties if tax is owed.

Paper tax returns for the 2025/26 tax year must be submitted by 31 October 2026, while online returns must be filed by 31 January 2027. Any tax owed, along with the first payment on account for the 2026/27 tax year, must also be paid by this date.

The reminder comes as the Self Assessment system continues to undergo significant change through the introduction of Making Tax Digital (MTD) for Income Tax.

From 6 April 2026, MTD for Income Tax became mandatory for sole traders and landlords with qualifying income above £50,000. The regime requires affected taxpayers to maintain digital records and submit quarterly updates to HMRC rather than relying solely on an annual tax return.

The first quarterly reporting deadline passed on 7 August 2026, with the next falling on 7 November 2026.

Zoe added: “Many people found themselves unprepared for MTD and, while there is a soft-landing period during the first year, compliance remains a legal requirement for those within the scope of the rules.”

She also highlighted that the qualifying income threshold will reduce to £30,000 from April 2027 and £20,000 from April 2028, bringing many more taxpayers into the regime over the next two years.

Zoe said: “If you have started freelancing, acquired a rental property or begun receiving income that has not already been taxed, now is the time to tell HMRC, rather than waiting until January.

“For those who are new to Self Assessment, there is already a great deal to understand, and the introduction of MTD adds a further layer of complexity. Taking advice early can help ensure your tax affairs are reported accurately and efficiently.”

Milsted Langdon is encouraging anyone who is unsure whether they need to register for Self Assessment, or how Making Tax Digital may affect them, to seek professional advice well ahead of the October deadline.

Each year, the firm’s tax specialists support hundreds of taxpayers across the South West and beyond, helping them meet their compliance obligations while identifying legitimate tax planning opportunities that could reduce their liabilities.

For more information about Milsted Langdon’s tax services, please get in touch.