Self Assessment Deadlines You Cannot Afford to Miss

Missing a Self Assessment deadline is one of the most expensive mistakes a sole trader or landlord can make. The penalties are automatic, they stack up quickly, and HMRC has no sympathy for people who simply forgot. Here is every deadline that matters for the current and upcoming tax years, and what happens if you miss them.

The tax year in the UK runs from 6 April to 5 April. For the 2025-26 tax year, you need to register for Self Assessment by 5 October 2026 if you have not already. If you are newly self-employed or have a new source of untaxed income, this registration deadline is your first obligation. Miss it and you are already behind.

The paper return deadline is 31 October 2026. Very few people file paper returns anymore, but if you do, this is your cutoff. The online filing deadline is 31 January 2027. This is the date most people know, and the one that causes the annual rush of last-minute filings.

The payment deadline is also 31 January 2027. Your tax bill for 2025-26 is due on the same day as your return. If you owe more than £1,000, you will also need to make a payment on account — an advance payment towards next year's tax bill. The first payment on account is due on 31 January and the second on 31 July.

Now here is where MTD changes things. From April 2026, if you are above the £50,000 threshold, you also need to submit quarterly updates. These are due by the 7th of the month following the end of each quarter. For the standard tax year, that means updates due by 7 August, 7 November, 7 February, and 7 May. Your final declaration replaces the traditional Self Assessment return and is due by 31 January following the end of the tax year.

The penalties for late filing are structured under HMRC's new points-based system. Each late submission earns you a point. Once you reach the threshold — which is two points for annual obligations and four points for quarterly — you receive a £200 penalty. After that, each subsequent late submission attracts another £200. The points expire after a period of compliance, but they accumulate faster than you might expect if you are filing quarterly.

Late payment penalties work differently. If your tax is more than 15 days late, you are charged a penalty of 2% on the amount outstanding. After 30 days, a further 2% is charged. After that, an annualised rate of 4% applies on the outstanding balance. Interest is charged on top from the day after the due date.

The practical impact of these penalties is significant. A sole trader who misses two quarterly updates and pays their tax two months late could easily face penalties of several hundred pounds, plus interest. These are not hypothetical numbers — they are the direct consequence of poor record-keeping and missed deadlines.

The best protection is simple: keep your records up to date and automate your submissions. Software like Accounted tracks your deadlines and can submit directly to HMRC, removing the risk of manual errors and forgotten dates. The cost of the software is a fraction of a single late filing penalty.

If you have already missed a deadline, act immediately. File as soon as possible to minimise penalties. If you have a reasonable excuse — genuine illness, bereavement, fire, flood — you can appeal. But "I forgot" or "I was too busy" are not reasonable excuses in HMRC's view.

Planning ahead is everything. Set calendar reminders for every deadline. Better yet, use software that reminds you automatically. Build quarterly reviews into your routine so that filing becomes a five-minute job rather than a weekend of panic. The businesses that treat deadlines seriously are the ones that avoid unnecessary costs and keep their relationship with HMRC clean.