Most people leave their tax return until January. You don't have to. You can file your 2025/26 self assessment from the very first day of the new tax year, and getting it done early is one of the simplest ways to take pressure off your finances.
This guide explains the exact date the door opens, every deadline that follows, and why filing sooner is genuinely better for most sole traders, contractors and landlords. It's written for anyone who has to send a return for the 2025/26 tax year, whether it's your first time or your tenth.
What's the earliest you can file your 2025/26 return? {#earliest}
The 2025/26 tax year runs from 6 April 2025 to 5 April 2026. You can file your return for that year from 6 April 2026 onwards, the day after the year ends.
That's nearly ten months before the online filing deadline of 31 January 2027. If you're organised, there's nothing stopping you from getting it done in April.
Why is 6 April the earliest date?
UK tax years run from 6 April to 5 April the following year, a quirk that dates back to calendar changes in the 1750s but is now baked into the system. You can only file a return once the tax year it covers has finished, so the moment 5 April passes, the year is complete and you're free to submit.
HMRC's online self assessment service handles the new year's returns from 6 April. Before that, there's simply no completed year to report.
What are the self assessment deadlines for 2025/26? {#deadlines}
Knowing every date up front lets you plan both your filing and your payment. Here's the full picture for 2025/26.
| Deadline | Date | What it's for |
|---|---|---|
| Earliest filing | 6 April 2026 | First day you can submit your 2025/26 return |
| Register for self assessment | 5 October 2026 | If 2025/26 is your first return |
| Paper return | Midnight 31 October 2026 | Deadline for paper filing |
| Online return | Midnight 31 January 2027 | Deadline for online filing |
| Balancing payment | Midnight 31 January 2027 | Pay any tax owed in full |
| Second payment on account | 31 July 2027 | If payments on account apply |
These dates come straight from HMRC's self assessment deadlines guidance.
Registration deadline: 5 October 2026
If you need to send a return for 2025/26 but didn't have to last year, you must register for self assessment by 5 October 2026. Miss it and you could face a penalty, even if you end up owing no tax. Registration isn't instant, so don't leave it that late.
Paper return deadline: 31 October 2026
Paper returns are due by midnight on 31 October 2026, three months earlier than online. The earlier cut-off gives HMRC time to process them. Most people now file online instead.
Online return and payment: 31 January 2027
The online filing deadline is midnight on 31 January 2027, and it's the same date any tax you owe must be paid. Hit both and you avoid the automatic penalties.
If your bill is over £1,000, you may also need to make payments on account, advance instalments towards the following year's tax. The first falls on 31 January (alongside your balancing payment) and the second on 31 July 2027.
You can estimate where you'll land with our self-employed tax calculator before you file.
Why is it better to file your tax return early? {#why-early}
Get help with your tax return →
Filing in spring instead of the depths of January has real, practical upsides.
You know your bill months in advance. Submit early and the exact figure is settled, so you can plan around it instead of guessing.
You have time to claim everything. Rushing in January is how legitimate expenses get missed. Filing early means you can review your figures properly and claim every allowable cost you're entitled to.
Your cash flow is easier to manage. If you owe £3,000 and file in April, setting aside £300 a month is far gentler than finding the lot in January.
You skip the January rush. HMRC's systems get very busy in late December and January, and so do accountants. File early and you avoid the bottleneck on both fronts.
It's less stressful. Ticking off a major annual job leaves you free to get on with running your business.
If you're a landlord, our guidance for landlords covers the property-specific points worth checking before you submit.
Will you get your tax refund sooner if you file early? {#refund}
If you're due money back, yes, filing early usually means getting it faster.
You might be owed a refund if you've overpaid tax during the year. This is common for CIS subcontractors, people with more than one job, and anyone who's had tax taken at source but has allowable expenses to set against it. HMRC processes any refund after you file and they've checked the return.
The logic is simple: the sooner you file, the sooner HMRC can process the return and release the refund. File in April and the money could be back within weeks; wait until January and you won't see it until the following spring. Straightforward returns are typically processed within a few weeks, though anything flagged for checking takes longer.
Illustrative example: a CIS subcontractor filing early Sam works as a CIS subcontractor in construction. During 2025/26, contractors deducted £4,500 in tax from his payments. After his actual liability and allowable expenses are worked out, his real bill is £3,000, so he's owed £1,500 back. Sam files on 10 April 2026, days after the service opens. If there are no queries, the refund could reach his account within a few weeks. Had he waited until January 2027, that £1,500 would have sat with HMRC for the best part of a year.
CIS subcontractors tend to benefit most, because tax is deducted at source by contractors and the deductions often exceed the real liability once expenses are counted. If that's you, our guidance for CIS contractors walks through what to gather.
Do you have to pay straight away if you file early? {#pay-early}
No. This is the worry that keeps people from filing early, and it's misplaced.
The filing date doesn't change the payment date. Whenever you submit, your tax is still due by 31 January following the end of the tax year. Filing early does not mean paying early.
You can pay early if you'd rather. Some people like clearing the bill the moment they know it; others prefer to hold the cash until the deadline. Both are fine.
You get longer to budget. File in April owing £4,000 and you've got nine months to set the money aside, which makes a large bill far more manageable.
Payment plans are easier to arrange. If you expect to struggle, HMRC's Time to Pay arrangements let you spread the cost. Filing early gives you room to set one up calmly rather than scrambling at the end of January.
The one thing that matters is that your payment reaches HMRC by midnight on 31 January. File when you like; just pay on time.
How long does it take to register for self assessment? {#register}
You have to be registered before you can file, and registration isn't instant, which is another reason to start early.
When you register online, HMRC verifies your identity and posts you what you need. Your Unique Taxpayer Reference (UTR) and activation details arrive by post, so allow a couple of weeks. Check HMRC's current timescales when you register, as they vary.
You'll receive:
- A Unique Taxpayer Reference (UTR)
- The details you need to activate your online account
- Information on accessing your self assessment account
The registration deadline is 5 October 2026, but registering in spring or summer is far safer. If you filed last year you're already registered, so just log in with your existing credentials. Your UTR stays the same every year, so keep it somewhere safe.
What's changing with Making Tax Digital? {#mtd}
Making Tax Digital (MTD) for Income Tax is HMRC's move to digital record-keeping and quarterly updates instead of a single annual return. It's already in place for VAT, and the Income Tax version begins to roll out from April 2026.
Who is affected first?
From 6 April 2026, MTD for Income Tax applies to sole traders and landlords with qualifying income over £50,000 (based on the 2024/25 tax year). If your qualifying income is below that, you're not in scope yet and you carry on filing annual returns as normal.
The threshold then steps down: from 6 April 2027 it covers qualifying income over £30,000 (based on 2025/26), and from 6 April 2028 it covers qualifying income over £20,000 (based on 2026/27).
How does MTD change reporting?
Instead of one annual return, those in scope keep digital records and send quarterly updates through compatible software, then a final declaration to wrap up the year.
| Aspect | Current system | MTD for Income Tax |
|---|---|---|
| Reporting frequency | Annual return | Quarterly updates plus a final declaration |
| Records | Any format | Must be digital |
| Software | Optional | Compatible software required |
| Who's affected | All self assessment taxpayers | Sole traders and landlords above the relevant threshold, phased from April 2026 |
Does MTD affect your 2025/26 return?
No. Your 2025/26 return, due by 31 January 2027, is filed under the current annual system. MTD only changes how the 2026/27 tax year onwards is reported for those in scope. So you should still aim to file 2025/26 early in the usual way, and use the lead time to get digital records in order if MTD will apply to you. Our self-assessment service covers both the current return and the transition.
Can you change your return after submitting it? {#amend}
Yes. If you spot a mistake, you can amend an online return within 12 months of the filing deadline. For a 2025/26 return, that means you have until 31 January 2028.
To do it, log in to your HMRC self assessment account, open the submitted return and update the figures. If you can't amend online, you can write to HMRC instead.
If the change increases your tax, you'll pay the extra and interest may run from the original due date. If it reduces your tax, HMRC refunds the difference, which takes a few weeks. Significant errors or unusual patterns can prompt an enquiry, so it pays to be accurate and correct genuine mistakes promptly. Better still, give yourself time to get it right first time, which is exactly what early filing buys you.
What happens if you miss the deadline? {#miss}
Missing the deadline gets expensive quickly. The penalties below are set out in HMRC's self assessment penalties guidance.
Late filing:
- An initial £100 penalty, even if you owe no tax
- After 3 months: £10 a day, up to a maximum of £900
- After 6 months: a further 5% of the tax due or £300, whichever is greater
- After 12 months: another 5% or £300, whichever is greater
Late payment (separate from filing penalties):
- 5% of the tax unpaid at 30 days
- A further 5% at 6 months
- A further 5% at 12 months
- Plus interest on the outstanding amount from the original deadline
Illustrative example: filing six months late owing £3,000 You'd face the £100 initial penalty, up to £900 in daily penalties, and a 6-month penalty of the greater of £300 or 5% of £3,000 (£150), so £300. That's up to £1,300 in filing penalties alone, before any late-payment penalties and interest if the tax is also unpaid.
HMRC may cancel a penalty if you have a reasonable excuse, such as serious illness, a bereavement or a technical failure of their own systems. Simply forgetting or being busy doesn't qualify. Filing well before January removes the risk entirely.
How can Zmartly help with your self assessment? {#zmartly}
Self assessment doesn't have to be a January scramble. At Zmartly, we keep your records straight through the year, so when filing season comes round everything's ready to go.
We work to claim every allowable expense you're entitled to, prepare your return carefully to reduce the risk of errors and enquiries, and encourage early filing so you get refunds faster or have months to budget for what you owe. If 2025/26 is your first return, we'll handle registration in good time. And as MTD for Income Tax rolls out, we'll get your digital records and compatible software set up so the switch is painless.
Want your 2025/26 return filed early and stress-free? Talk to a Zmartly accountant and get it sorted well before the January rush.
Frequently asked questions {#faqs}
What's the earliest you can file your 2025/26 self assessment?
You can file from 6 April 2026, the day after the 2025/26 tax year ends. That gives you nearly ten months before the 31 January 2027 online deadline.
Do you have to pay your tax bill immediately if you file early?
No. Filing early doesn't change your payment date. Whenever you submit, any tax for 2025/26 is due by midnight on 31 January 2027. Early filing just gives you longer to budget.
Will you get your tax refund faster if you file early?
Usually, yes. HMRC processes straightforward returns within a few weeks of submission. Filing in April or May 2026 means a refund could arrive in the summer, rather than the following spring if you wait until January.
When is the deadline to file your 2025/26 self assessment?
The online deadline is midnight on 31 January 2027. Paper returns are due by 31 October 2026. Any tax owed must also be paid by 31 January 2027.
Can you change your self assessment return after filing?
Yes. You can amend an online return within 12 months of the filing deadline. For 2025/26, that means until 31 January 2028, either through your HMRC online account or by writing to HMRC.
What happens if you miss the self assessment deadline?
You get an automatic £100 penalty even if you owe no tax. After 3 months it's £10 a day up to £900, then at 6 and 12 months a further 5% of the tax due or £300 each time, whichever is greater. Paying late triggers separate 5% penalties at 30 days, 6 months and 12 months, plus interest.
Does Making Tax Digital affect my 2025/26 return?
No. Your 2025/26 return is filed under the current annual system. MTD for Income Tax starts to apply from 6 April 2026 for sole traders and landlords with qualifying income over £50,000 (based on 2024/25), and only changes how the 2026/27 tax year onwards is reported.
