Self Assessment Tax Return Deadlines You Can't Afford to Miss in 2027

If you're a sole trader, a landlord with rental income, or a partner in a business partnership, chances are you'll need to file a Self Assessment tax return with HMRC. It's one of those tasks that's easy to put off — but miss the deadline, and HMRC won't hesitate to issue automatic penalties, even if you don't owe a penny in tax.

Here are the key dates for the 2025/26 tax year that you need to have on your radar:

5 October 2026 — If you've received taxable income for the first time during the 2025/26 tax year and haven't yet registered for Self Assessment, this is your deadline to notify HMRC. Once registered, you'll receive a Unique Taxpayer Reference (UTR), which you'll need to file your return.

31 October 2026 — The deadline for submitting a paper Self Assessment return for the 2025/26 tax year. Paper filers are a shrinking group, but if that's your preference, don't leave it to the last minute.

31 January 2027 — This is the big one. It's the deadline for filing your online Self Assessment return for the 2025/26 tax year, and it's also the date by which any tax you owe must be paid. Miss either, and the penalties start stacking up.

Getting your return filed early doesn't mean you have to pay early — it just means you know exactly where you stand, with no nasty surprises in January.

Important: Making Tax Digital for Income Tax from April 2026

If you are a sole trader or landlord with gross income above £50,000, the 2025/26 tax year is your first year under Making Tax Digital for Income Tax (MTD for ITSA). Instead of a single annual return, you are now required to submit quarterly updates to HMRC throughout the year using compatible software, followed by a final declaration. The final declaration deadline remains 31 January 2027 — the same date as the traditional Self Assessment deadline — but your quarterly reporting obligations run throughout the year. Missing a quarterly deadline will trigger a penalty point under HMRC's new points-based system.

If your gross income is between £30,000 and £50,000, MTD for ITSA applies to you from 6 April 2027. If it is between £20,000 and £30,000, it applies from 6 April 2028.

At Companies999, our qualified self assessment accountants take the stress out of Self Assessment and MTD for ITSA. We'll prepare and submit your return, make sure you're claiming every allowable expense, and ensure you're only paying the tax you genuinely owe. No guesswork, no overpaying.

Get in touch with us for a straightforward, fixed-fee quote — and get your 2027 deadline sorted well ahead of time. If you're a sole trader or contractor, also see our specialist sector pages for packages tailored to your situation.