Understanding HMRC deadlines for Self Assessment and Corporation Tax

Navigating taxes can feel overwhelming for business owners, and the various deadlines set by HMRC can lead to confusion. Whether you’re a sole trader with a Self Assessment tax return or a limited company managing Corporation Tax, knowing these deadlines is crucial. Missing them can lead to penalties that add to the stress. This post gives a clear overview of the HMRC deadlines for Self Assessment and Corporation Tax, so you can stay on top of your tax obligations.

What is Self Assessment?

Self Assessment is the system HMRC uses to collect Income Tax from people who are self-employed, earn rental income, or have other income that is not taxed automatically. Under this system, taxpayers must report their income and work out how much tax they owe.

Key deadlines for Self Assessment

The examples below are for the 2025/26 tax year, which ran from 6 April 2025 to 5 April 2026.

1. Registering for Self Assessment: if you need to complete a tax return and HMRC doesn’t already know, you must tell HMRC by 5 October following the end of the tax year. For the 2025/26 tax year, that is 5 October 2026.
2. Filing your tax return: a paper return must reach HMRC by 31 October following the end of the tax year (31 October 2026 for 2025/26), and an online return must be filed by 31 January (31 January 2027 for 2025/26).
3. Paying your tax bill: any tax owed must be paid by 31 January (31 January 2027 for 2025/26). This includes the balancing payment for the tax year and, if they apply to you, the first payment on account for the next one.
4. Payments on account: these are advance payments towards your next tax bill, due on 31 January and 31 July, which spread the cost over the year. You don’t have to make them if your last tax bill was less than £1,000, or if you paid more than 80% of the tax you owed outside Self Assessment.

Consequences of missing Self Assessment deadlines

Missing these deadlines can be expensive. If your return is late, there is an automatic £100 penalty. After three months, daily penalties of £10 apply, up to £900. After six months, and again after twelve, there is a further penalty of 5% of the tax due or £300, whichever is greater. Paying late also costs you: interest is charged, plus penalties of 5% of the unpaid tax at 30 days, 6 months and 12 months. Staying organised is the best way to avoid these unnecessary costs.

If you use Making Tax Digital for Income Tax, the deadlines for your tax return and payments stay the same; read our guide to Making Tax Digital for Income Tax for the quarterly updates.

What is Corporation Tax?

Corporation Tax is charged on the profits of limited companies and certain other organisations. It differs from Self Assessment, which is for individuals. Understanding the Corporation Tax deadlines is essential for business owners to stay compliant and avoid penalties.

Key deadlines for Corporation Tax

The examples below use a company whose accounting period ends on 31 March 2026.

1. Registering for Corporation Tax: you must tell HMRC that your company is active within three months of the start of its first accounting period for Corporation Tax (usually when it starts doing business).
2. Paying your Corporation Tax: Corporation Tax must be paid 9 months and 1 day after the end of your accounting period. For a period ending 31 March 2026, payment is due by 1 January 2027. (Companies with taxable profits over £1.5 million pay in instalments instead.)
3. Filing your Company Tax Return: the deadline for the Company Tax Return (CT600) is 12 months after the end of the accounting period. For a period ending 31 March 2026, the return must be filed by 31 March 2027.

Consequences of missing Corporation Tax deadlines

If your Company Tax Return is late, there is a £200 penalty, and another £200 once it is three months late. After six months HMRC estimates your Corporation Tax bill and adds a penalty of 10% of the unpaid tax, with another 10% after twelve months. If your return is late three times in a row, the £200 penalties rise to £1,000 each. Paying late also means interest charges that eat into your business finances.

Important considerations for business owners

As a business owner, especially in Belfast and Northern Ireland, it’s vital to understand the deadlines that apply to your situation. Here are some practical ways to manage your tax obligations more effectively:

1. Keep accurate records: organised financial records make your Self Assessment or Corporation Tax return much simpler. Accounting software can streamline the process and give you a clear view of your finances.
2. Set reminders: use calendars and reminder apps to flag upcoming deadlines. Planning ahead avoids the stress of last-minute submissions and potential penalties.
3. Seek professional advice: if you are unsure about your obligations or deadlines, an accountant can guide you, make sure you follow HMRC’s rules and help you plan ahead.

Final thoughts

Knowing the HMRC deadlines for Self Assessment and Corporation Tax is essential for business owners in Belfast and across Northern Ireland. By staying informed and organised, you can stay compliant and avoid unnecessary penalties. Register on time, submit your returns punctually, and pay any tax owed by the deadline. With careful planning, you can handle your tax responsibilities confidently and focus on what truly matters: growing your business.

Need a hand with your deadlines? See our Self Assessment and Corporation Tax services, or book a free consultation.