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

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

Although this part of the tax calendar is generally quieter, missed filings or payments can still have consequences if they remain unresolved.

Key Tax and Payroll Deadlines for August and September 2025

• 1 August 2025 – Due date for corporation tax due for the year ended 31 October 2024.
• 19 August 2025 – PAYE and NIC deductions due for month ended 5 August 2025 (If you pay your tax electronically the due date is 22 August 2025)
• 19 August 2025 – Filing deadline for the CIS300 monthly return for the month ended 5 August 2025.
• 19 August 2025 – CIS tax deducted for the month ended 5 August 2025 is payable by today.
• 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.

Why Late-Summer Deadlines Are Still Relevant

Tax obligations continue throughout the year. Even where there is no single headline deadline, routine filings and payments may still fall due during August and September.

If these obligations were missed, HMRC may still treat them as outstanding until they are resolved, sometimes identified later during a review carried out by a tax specialist in London or through HMRC’s own checks.

Common Obligations Around August and September

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 August or September 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 may still appear later through HMRC statements, reminders, or compliance checks, often prompting taxpayers to seek input from a tax consultant in London to clarify their position.

Why It Is Worth Reviewing Now

Because August and September tend to be quieter months, routine obligations are sometimes overlooked. Addressing any missed items sooner can help prevent issues from accumulating later in the tax year.

Reviewing this period can also make it easier to stay organised ahead of busier deadlines later on.

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 August or September 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 Hammersmith and nearby areas such as Askew Village and Wood Lane, CIGMA Accounting can review your position and confirm whether anything remains outstanding or needs to be addressed.