Self-Assessment Deadlines Explained: A Family Business Guide

Meta Description: Self Assessment deadlines for 2025/26 explained. Learn key dates, HMRC penalties, and how your family business can avoid costly mistakes. Running a family business means juggling a lot of responsibilities—tax deadlines shouldn’t be one of the things you miss. Self-Assessment is how HMRC collects income tax from those with untaxed income, and family businesses must stay on top of it to avoid fines.

Key Deadlines for the 2024/25 Tax Year

5 October 2025 – Deadline to register for Self-Assessment if it’s your first time.
31 October 2025 – Deadline for paper tax returns.
31 January 2026 – Deadline for online filing and tax payment.
31 July 2026 – Deadline for second payment on account (if applicable).

Penalties for Missing Deadlines

1 day late: £100 fine, even if no tax is owed.
3 months late: £10 per day fine, up to £900.
6 months late: 5% of tax due, or £300 (whichever is greater).
12 months late: additional 5% of tax due, or £300.

From April 2025, fines get tougher: daily penalties increase to £20, and 6- and 12-month charges rise to 6% and 8% respectively.

Tips for Family Businesses

Assign one family member or your accountant to take responsibility.
Automate reminders—don’t rely on memory.
Put aside funds regularly so the January deadline doesn’t create a cashflow crisis.

Avoid fines and stress. Let RMC Accountants handle your Self Assessment while you focus on running your family business.

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