Tax Diary: September to October 2025 Deadlines That May Still Matter

This tax diary highlights key UK tax deadlines that fall between September and October 2025. It is intended for taxpayers reviewing whether all obligations from this mid-to-late tax year period were dealt with correctly.

Even where these dates have passed, missed filings or payments can still have consequences if they remain unresolved, particularly where no review was carried out at the time with a tax consultant in London or equivalent professional support.

Key Tax and Payroll Deadlines for August and September 2025

• 1 September 2025 – Due date for corporation tax due for the year ended 30 November 2024.
• 19 September 2025 – PAYE and NIC deductions due for month ended 5 September 2025. (If you pay your tax electronically the due date is 22 September 2025)
• 19 September 2025 – Filing deadline for the CIS300 monthly return for the month ended 5 September 2025.
• 19 September 2025 – CIS tax deducted for the month ended 5 September 2025 is payable by today.
• 1 October 2025 – Due date for Corporation Tax due for the year ended 31 December 2024.
• 19 October 2025 – PAYE and NIC deductions due for month ended 5 October 2025. (If you pay your tax electronically the due date is 22 October 2025)
• 19 October 2025 – Filing deadline for the CIS300 monthly return for the month ended 5 October 2025.
• 19 October 2025 – CIS tax deducted for the month ended 5 October 2025 is payable by today.
• 31 October 2025 – Latest date you can file a paper version of your 2024-25 self-assessment tax return.

Why September and October Deadlines Still Matter

Tax compliance continues throughout the year. Where an obligation was missed during September or October, HMRC may continue to treat it as outstanding until it is resolved.

Reviewing this period can help ensure that routine obligations were met and prevent issues carrying forward into later parts of the tax year.

Common Obligations Around September and October

Deadlines during this period often relate to:

• VAT return submissions and payments
• PAYE and National Insurance reporting and payments
• Ongoing Self Assessment compliance
• Employer reporting obligations

Which obligations applied will depend on your circumstances, such as whether you were self-employed, an employer, or VAT-registered.

What Happens If a Deadline Was Missed?

If a tax deadline in September or October 2025 was missed, HMRC may have charged interest from the original due date. In some cases, penalties can also apply depending on the type of obligation and how late it was dealt with.

Even where no immediate penalty notice was issued, unresolved issues can still appear later through HMRC statements, reminders, or compliance checks, sometimes highlighted when matters are reviewed by a tax advisor in London after the event.

Why It Is Worth Reviewing Now

Because September and October sit between busier points in the tax calendar, routine obligations are sometimes overlooked. Addressing any missed items now can help reduce the risk of problems later in the year.

Reviewing this period can also make it easier to stay organised ahead of upcoming deadlines.

HMRC Guidance on Late Tax Obligations

HMRC explains how late filings and payments are handled, including interest and penalties, on
GOV.UK.

Did You Miss Any September or October 2025 Deadlines?

Whether any action is required now will depend on which obligations applied to you at the time and whether they were dealt with correctly. For individuals and businesses in Fulham Broadway and nearby areas such as Munster Village and Fulham Reach, CIGMA Accounting can review your position and confirm whether anything remains outstanding or needs to be addressed.