Tax Diary: December 2025 to January 2026 Deadlines That May Still Matter

This tax diary highlights key UK tax deadlines that fall between December 2025 and January 2026. It is intended for taxpayers reviewing whether all filing and payment obligations around the year-end and January deadline were dealt with correctly.

January is one of the most important points in the tax calendar. Even where deadlines have passed, unresolved matters from this period can still result in penalties, interest, or HMRC follow-up action, particularly where issues were not identified at the time with the help of a tax consultant in London.

Key Tax and Payroll Deadlines for December 2025 and January 2026

• 1 December 2025 – Due date for Corporation Tax payable for the year ended 28 February 2025.
• 19 December 2025 – PAYE and NIC deductions due for month ended 5 December 2025. (If you pay your tax electronically the due date is 22 December 2025).
• 19 December 2025 – Filing deadline for the CIS300 monthly return for the month ended 5 December 2025.
• 19 December 2025 – CIS tax deducted for the month ended 5 December 2025 is payable by today.
• 30 December 2025 – Deadline for filing 2024-25 self-assessment tax returns online to include a claim for under payments to be collected via tax code in 2026-27.
• 1 January 2026 – Due date for Corporation Tax due for the year ended 31 March 2025.
• 19 January 2026 – PAYE and NIC deductions due for month ended 5 January 2026. (If you pay your tax electronically the due date is 22 January 2026).
• 19 January 2026 – Filing deadline for the CIS300 monthly return for the month ended 5 January 2026.
• 19 January 2026 – CIS tax deducted for the month ended 5 January 2026 is payable by today.
• 31 January 2026 – Last day to file 2023-24 self-assessment tax returns online.
• 31 January 2026 – Balance of self-assessment tax owing for 2024-25 due to be settled on or before today unless you have elected to extend this deadline by formal agreement with HMRC. Also due is any first payment on account for 2025-26.

Why Year-End and January Deadlines Still Matter

Tax obligations do not fall away once a deadline has passed. Where a return was not filed or a payment was not made on time, HMRC continues to treat the issue as outstanding until it is resolved.

January deadlines are particularly significant, as penalties are often triggered automatically and interest begins to accrue on unpaid tax, something a tax specialist in London will often flag when reviewing historic compliance.

Common Obligations Around December and January

Deadlines during this period often relate to:

• Self Assessment filing deadlines
• Payment of tax due for the previous tax year
• Payments on account for the current tax year, where applicable
• VAT return submissions and payments
• PAYE and National Insurance reporting and payments

Which obligations applied will depend on your circumstances, such as whether you were self-employed, a landlord, an employer, or otherwise required to submit a Self Assessment return.

What Happens If a Deadline Was Missed?

Missing a January deadline usually results in an automatic late filing penalty. Interest is also charged on any unpaid tax from the original due date. Further penalties can apply if delays continue. Even where no immediate action was taken by HMRC, outstanding issues may still be pursued later through statements, reminders, or compliance checks.

Why It Is Worth Reviewing Now

Some taxpayers assume that once a new year has started, missed deadlines are no longer relevant. In practice, unresolved issues often resurface later and can become more costly to deal with. Reviewing whether all December 2025 and January 2026 obligations were met can help prevent unexpected charges or enforcement action.

HMRC Guidance on Late Tax Deadlines

HMRC provides guidance on late filing, late payment, penalties, and interest on GOV.UK.

Did You Miss Any December 2025 or January 2026 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 Kingston upon Thames and nearby areas such as Long Ditton and Tolworth, CIGMA Accounting can review your position and confirm whether anything remains outstanding or needs to be addressed.