Self Assessment Deadlines and Penalties | Brookwood
The Self Assessment Deadlines and Penalties You Need to Know
Self Assessment deadlines look simple on paper. A few dates, a few forms, one January deadline everyone knows about. The problem is what happens when one of those dates slips.
A missed registration can lead to a late return. A late return can lead to penalties. A late payment adds interest and further surcharges. Before long, a manageable tax bill has picked up avoidable costs.
This guide covers the key Self Assessment deadlines for 2025/26, what triggers each penalty, how payments on account catch people out, and where Making Tax Digital now sits, given that the first phase is live.
All Self Assessment Deadlines at a Glance
The UK tax year runs from 6 April to 5 April. Once the 2025/26 tax year closes on 5 April 2026, there are five deadlines to keep track of:
5 October: register for Self Assessment if you're filing for the first time
31 October: submit a paper tax return
30 December: optional deadline to have underpaid tax collected through your PAYE tax code
31 January: online filing and payment deadline
31 July: second payment on account
Miss any of these and HMRC issues an automatic penalty. There is no grace period.
The 31 January deadline is the one most people know, and for good reason. Two obligations land on the same day: your online return and your tax payment. For 2025/26, that date is 31 January 2027.
The 30 December option is worth having in the diary too. If you owe less than £3,000 and you're employed or receiving a pension, filing by 30 December may allow HMRC to collect the tax through your PAYE code, spread across your pay rather than due in one lump sum in January.
One detail catches people every year: HMRC issues an automatic £100 penalty for a late return even when there is no tax to pay.
When to Submit Self Assessment for 2025/26
The 2025/26 tax year ended on 5 April 2026, so you can file now. Filing early doesn't mean paying early. The payment deadline remains 31 January 2027. What it does give you is clarity, especially if your income has shifted or you need to plan for payments on account.
The key dates in full:
5 October 2026: registration deadline for first-time filers with untaxed income in 2025/26
31 October 2026: paper return deadline, by midnight
30 December 2026: optional deadline to collect qualifying tax debts through PAYE
31 January 2027: online filing and payment deadline, by midnight
31 July 2027: second payment on account
If you've never filed a Self Assessment return before, don't leave registration until October is already on the horizon. HMRC needs time to issue your Unique Taxpayer Reference (UTR), the 10-digit number used for Self Assessment correspondence, and that can take up to ten working days.
Making Tax Digital: Now Live for Higher Earners
Making Tax Digital for Income Tax, or MTD ITSA, is no longer on the horizon. From 6 April 2026, sole traders and landlords with income over £50,000 must use MTD-compatible software and submit quarterly digital updates to HMRC.
The thresholds expand over the next two years:
April 2026, now live: sole traders and landlords earning over £50,000
April 2027: sole traders, landlords, and partnerships earning over £30,000
April 2028: sole traders and landlords earning over £20,000
If you're currently below the £50,000 threshold, your standard Self Assessment deadlines remain unchanged for now. But if you're approaching the next band, it is worth preparing before the rules apply to you.
What Changes in Practice
Instead of one return by 31 January, MTD filers submit four quarterly updates through compatible accounting software such as QuickBooks, Xero, FreeAgent, and others. A final declaration then confirms the full-year position.
For most business owners and landlords, the real adjustment is keeping records current throughout the year rather than catching everything up after 5 April. That discipline can be useful, but only if the system is set up properly from the start.
HMRC is still confirming the penalty regime for late MTD submissions, so it is worth watching for updates there.
The 5 October Registration Deadline
If you're filing Self Assessment for the first time, you must register with HMRC by 5 October following the end of the tax year in which the income arose. For 2025/26, that means 5 October 2026.
This deadline applies whenever your circumstances change. Common triggers include:
Self-employment income above the £1,000 trading allowance
Dividends above the dividend allowance
Total income over £100,000
Untaxed foreign income
Missing the 5 October deadline doesn't make the tax go away. HMRC can assess what you owe and apply penalties retrospectively. The penalty for failing to notify is calculated as a percentage of unpaid tax, not a fixed fine.
Timing and behaviour matter. If you register late but before 31 January, and you pay the tax on time, unprompted disclosure can reduce the penalty significantly. If you're already registered from a previous year, there's nothing further to do.
What Happens If You Miss the 31 October Paper Deadline?
A paper return received after 31 October is treated as late. HMRC issues an automatic £100 penalty, regardless of whether any tax is owed.
The simplest fix is switching to online filing, which extends the deadline to 31 January. You'll need an HMRC online account to do this. If you've never filed online before, allow several days for HMRC to issue your Government Gateway credentials.
Paper filing is now uncommon. Fewer than 3% of Self Assessment returns are submitted on paper, and the extra three months that online filing gives you is a clear reason to switch.
There is also a knock-on effect. If you want HMRC to collect a small tax debt through your PAYE code, that option requires filing by 30 December. A late paper return puts that option under pressure before January even arrives.
Payments on Account: The Part That Catches People Out
Payments on account are the biggest surprise for many first-year Self Assessment filers. When you submit your first return, you may have to pay the full tax bill for the year just ended, plus an advance payment of 50% of that bill towards the following year, all by 31 January.
That first payment can feel like 150% of the tax you were expecting. It is, technically. And it is common enough that it should be planned for, not discovered.
How They Work
Each payment on account is 50% of your previous year's Self Assessment bill. If that bill was £2,000, you pay an additional £1,000 by 31 January and another £1,000 by 31 July.
The payment dates are:
31 January: balancing payment for the previous year, plus the first payment on account for the current year
31 July: second payment on account for the current year
If you expect your income to fall, you can apply to reduce your payments on account before they're due. Reduce them too far, and HMRC charges interest on the shortfall from the original payment date.
Collecting Tax Through PAYE
If you owe less than £3,000 and you're employed or receiving a pension, HMRC may be able to collect the tax by adjusting your PAYE code, spread across your pay rather than paid in one go. To use this option, you need to file by 30 December.
What If You Can't Pay on Time?
HMRC charges daily interest from the day after the payment deadline on unpaid tax. If you can't pay in full, setting up a Time to Pay arrangement before the deadline can stop late payment surcharges from building, though interest continues to run.
Penalties for Missing Self Assessment Deadlines
Late filing penalties escalate quickly:
Day 1 late: £100 fixed penalty, even with no tax owed
3 months late: £10 per day for up to 90 days, with a maximum of £900
6 months late: an additional 5% of the tax due or £300, whichever is greater
12 months late: a further 5% of the tax due or £300, whichever is greater. In deliberate cases, this can reach 100% of the tax owed
A return filed twelve months late with no tax due can still accumulate more than £1,600 in fixed charges before interest is counted.
Late Payment Penalties
These are separate from filing penalties and apply when tax remains unpaid after the deadline:
30 days after the deadline: 5% of the unpaid tax
6 months after the deadline: a further 5%
12 months after the deadline: a further 5%
Daily interest runs from the day after the payment deadline on top of all this.
Reasonable Excuse
HMRC can cancel or reduce penalties where there is a reasonable excuse, such as serious illness, bereavement, or an HMRC system failure. Not knowing the deadline generally doesn't qualify.
HMRC will also look at whether you acted reasonably once the problem came to light. The excuse needs to explain the delay, and you need to have put things right as promptly as the circumstances allowed.
Frequently Asked Questions
What is the Self Assessment deadline for 2025/26?
The online filing and payment deadline for 2025/26 is 31 January 2027. The paper return deadline is 31 October 2026. First-time filers need to register with HMRC by 5 October 2026.
What happens if I miss the Self Assessment deadline?
HMRC issues an automatic £100 penalty on day one, even if no tax is owed. Penalties escalate at three months, six months, and twelve months. Late payment also attracts separate surcharges and daily interest.
Can I reduce my payments on account?
Yes, if you expect your income to be lower than the previous year. If you reduce them too far and underpay, HMRC will charge interest on the shortfall.
What if I can't pay my Self Assessment bill?
Contact HMRC before the deadline and ask about a Time to Pay arrangement. This can prevent late payment surcharges from accruing, though interest will still run.
Does Making Tax Digital affect my Self Assessment filing?
From April 2026, sole traders and landlords earning over £50,000 are now required to file quarterly digital updates using MTD-compatible software. The threshold drops to £30,000 in April 2027 and £20,000 in April 2028.
Leave the Deadlines to Us
At Brookwood, we work with sole traders, landlords, and company directors across Oxfordshire. Whether you've had a penalty notice, missed a deadline, or just want to get your 2025/26 filing under control, contact us and we'll help you understand exactly where you stand.
