If you’re self-employed, a landlord, in a partnership or otherwise required to complete a Self Assessment tax return, HMRC gives you several self assessment deadlines to hit each year, and they’re not restricted to January.
The risk of missing these deadlines involves being entered into HMRC’s penalty system, where penalties start adding up from day one and keeps climbing the longer a return or payment is outstanding.
This guide brings together everything you need to know about Self Assessment tax returns in one place. This includes:
- Deadlines that apply to you
- Exactly how HMRC’s late filing and late payment penalties are calculates
- The payment date many taxpayers forget about
- A complete checklist of documents you’ll need to get your return done properly and on time.
Quick answer: The two deadlines that matter to almost everyone is 31st January (submit your online return and pay any tax owed) and 31st July (your second payment on account, if you make them). Miss either one and penalties and interest start building immediately. Read on for the full calendar, the penalty breakdown, and a document-by-document checklist.
To get started with your Self Assessment before the next deadline, speak to our expert team of tax accountants.
The Self Assessment tax year and filing period
The tax year runs from 6th April to 5th April every year. So, the return you’re preparing now covers everything you earned between 6th April 2025 and 5th April 2026. This is usually referred to as the “2025/26 tax return.”
You can start filing as soon as the tax year ends, and the earlier you file, the earlier you know what you owe and the longer you can budget for it.
Key Self Assessment dates you need to know
Read below the key Self Assessment deadlines from HMRC for the entire tax year, and what you need to know about each one.
| Deadline | What it’s for |
| 5th October | Register for Self Assessment if you’re new to it, or need to file for the first time in a while |
| 31st October
|
Paper tax returns must reach HMRC |
| 30th December | Online returns must be filed if you want HMRC to collect what you owe through your tax code (‘coding out’), rather than paying it as a lump sum |
| 31st January | Online tax returns must be submitted, and any tax owed for the year must be paid |
| 31st July | Second payment on account, if HMRC has asked you to pay in instalments |
A few things worth understanding about Self Assessment:
- If you’re new to Self Assessment, the 5th October deadline is the one to add to the diary first. If you miss it, you may still owe tax by 31st January even though HMRC hasn’t formally told you what to file.
- Paper returns have an earlier deadline than online returns. Almost everyone files online now, partly because of this three-month head start it gives you.
- The 30th December deadline only matters if you want tax collected via PAYE rather than paid directly. It’s useful if you’re employed as well as self-employed and want to spread the cost.
- 31st January is a double deadline, as it’s both the day your return is due and the day your payment is due. Filing on time doesn’t help if you haven’t also paid.
The Self Assessment payment most people forget about: Payments on account (31st July)
This is the one that catches people out, because it doesn’t feel like “tax season” the way January does, and there’s rarely a letter through the door drawing attention to it in the same way.
If your last Self Assessment bill was over £1,000, and less than 80% of your tax is already collected at source (through PAYE, for example), HMRC will usually ask you to make payments on account. This is advance payments towards your next year’s tax bill, based on what you owed the year before.
How it works:
- For the self assessment deadlines on 31st January, you pay three things at once. You pay the balance for the year just finished, plus the first of two advance payments (each worth half of your previous year’s bill) towards the year ahead.
- The second advance payment then falls due on 31st July, six months later, with no accompanying filing deadline, no return to submit, just a bill sitting on your HMRC account.
Miss it, and interest starts building from 1st August at HMRC’s current late payment rate.
If your income has dropped, you don’t have to pay the full amount. HMRC lets you apply to reduce your payments on account if you genuinely expect to earn less than the year before.
The catch is that if you reduce it too far and turn out to be wrong, you’ll be charged interest on the shortfall. This means you’ll need a proper look at your numbers. If your income has changed and you’re not sure what to do about your payments on account, get in touch with our team before these self assessment deadlines land, not after.
What happens if you miss a Self Assessment?
HMRC charges separate penalties for filing late and for paying late. It’s entirely possible to be hit with both at once.
Penalties for filing your return late
| How late the filing is | Penalty |
| Immediately after the deadline | A fixed £100 penalty, charged even if you don’t owe any tax. |
| More than 3 months late | An additional £10 per day, up to 90 days (a maximum of £900). |
| More than 6 months late | A further 5% of the tax due, or £300 (whichever is greater). |
| More than 12 months late | Another 5% of the tax due, or £300 (whichever is greater). In the most serious cases (where HMRC judges the withholding to be deliberate), this can rise to 100% of the tax due. |
If you file a partnership return late, every partner is charged a penalty, not just the one who was responsible for filing.
Penalties for paying your tax bill late
Separately from filing penalties, HMRC charges the following on any tax paid late:
- 5% of the unpaid tax at 30 days
- A further 5% at 6 months
- A further 5% at 12 months
On top of these fixed penalties, interest is charged daily on any tax and any penalties that remain unpaid, for as long as the balance is outstanding. This is separate from the percentage penalties above and continues to accrue until the bill is cleared.
Registering late
If you register for Self Assessment after 5th October and still haven’t paid what you owe by 31st January, you may also face a ‘failure to notify’ penalty, calculated based on the tax you still owe.
If you disagree with a penalty
If you have a reasonable excuse, a genuine, unavoidable reason you couldn’t file or pay on time, you can appeal. HMRC decides these on a case-by-case basis, so it’s worth getting advice before you submit an appeal rather than after it’s been rejected.
Can you amend a Self Assessment tax return?
Yes, you can. Mistakes happen, and HMRC has a formal process for correcting them.
If you filed online, you could go back in and amend your return within 12 months of the original self assessment deadlines. For most people filing the 2025/26 return, that means corrections need to be made by 31st January 2028. You’ll need to wait 72 hours after your original submission before the amendment option becomes available, then sign back into your HMRC account and re-submit the corrected sections.
If you filed on paper, you could send amended pages to HMRC, and write “amendment” clearly on each page, along with your name and Unique Taxpayer Reference (UTR).
What happens next depends on the correction:
- If the change means you owe more tax, your updated bill will show the extra amount, the new payment deadline, and any effect on your payments on account.
- If the change means you’re owed money back, you may be able to claim a refund.
If you’ve missed the 12-month window, or the return relates to an earlier tax year, you can’t amend it directly online, you’ll need to write to HMRC instead, either to report income you left off the original return or to claim overpayment relief (which is possible up to 4 years after the end of the relevant tax year).
Getting a return right the first time is always cheaper and less stressful than correcting it later, which is exactly what a Self Assessment tax accountant is there to help you avoid.
The Self Assessment checklist for documents you need to submit on time
The single biggest cause of late or rushed returns is simply not having everything to hand when you sit down to file. Work through this checklist a few weeks before your filing date, not the night before, to meet all self assessment deadlines.
Employment and directorships
- P60 for the tax year (or P45 if you left a job partway through)
- Details of any Benefits in Kind provided by an employer
- PAYE reference number for each employer, if you had more than one.
Pension income
- P60 or certificate of pension paid, for any occupational pension
- Your state pension notification letter
- Pension statements from any private pension provider
- Records of any payments you’ve made into a pension, so you don’t miss out on tax relief.
Self-employment and partnerships
- Full income and expenditure figures from your bookkeeping records
- A separate set of figures for each self-employment or partnership, if you have more than one.
Investment and other income
- Dividend or distribution vouchers for shares, unit trusts, or company dividends
- Interest certificates from banks and building societies
- Income and expenditure records for any rental property, including mortgage interest statements
- R185 certificates for income from trusts, settlements, or estates
- Chargeable Event Certificates for any life assurance policies or bonds cashed in
- Documentation for any overseas income.
Outgoings that may reduce your tax bill
- Records of tax-deductible employment expenses (professional subscriptions, travel, etc.)
- Gift Aid or Deed of Covenant payment details
- Statements showing interest paid on qualifying loans and mortgages
- Student Loans Company statement, if applicable.
Capital gains
- Records of anything sold, gifted, swapped, or otherwise disposed of during the year, including purchase price, sale price, and dates, if it might trigger Capital Gains Tax.
Administration
- Your Unique Taxpayer Reference (UTR) and Government Gateway login
- Bank account details, if you’re due a refund.
If you’re missing any of the above, chase it up now. HMRC won’t accept “I’m waiting on a document” as a reason to file late.
How Self Assessment tax accountants will help you beat the deadline
Filing on time is only half the job. Filing correctly on time is what protects you from penalties, interest, and the HMRC enquiry that follows an incomplete return. A Self Assessment tax accountant does more than fill in boxes. They will:
- Work out exactly what you owe, including your payments on account, so there are no surprises in January or July
- Flag reliefs and allowances you’re entitled to, including pension contributions and Gift Aid, that are easy to miss without experience
- Handle corrections properly if HMRC queries something, instead of leaving you to interpret the letter yourself
- Keep a record trail in case HMRC opens an enquiry, which can be made up to 12 months after your filing date.
Need help with Self Assessment tax returns or facing HMRC penalties?
At Tax Driven Accountants, our Self Assessment tax return service covers exactly this. It includes preparing and submitting your return, as well as advising on payments on account and supporting you through any HMRC enquiry, for a fixed fee agreed in advance.
With 15 physical offices and 25 service areas in the UK, you can work with a local accountant in person or manage everything digitally, whichever suits you.
Let our local qualified tax accountants handle the paperwork and meet self assessment deadlines so you can focus on running your business.
Contact us on 0800 999 1800 or send us an enquiry.
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This is not an exhaustive list and should be considered as a guide only, and the guidance was up to date upon it’s release on 27th August 2026.
