The online Self Assessment return for the 2025 to 2026 tax year, the balancing payment for that year, and the first payment on account for 2026 to 2027 are all due by 11:59pm on 31 January 2027. Filing early does not make the payment due earlier, which is why October is the right month to deal with it.
Every January the same thing happens, and it is not really about tax. It is about three months of small jobs being left until the week one big one is due. The records, the login, the allowable expenses, the figure, the money to pay the figure. None of those has to happen in January.
What follows is the run up, month by month, and the two dates that matter besides the obvious one.
What are the Self Assessment deadlines for the 2025 to 2026 return?
- 5 October 2026. Registration deadline for anyone filing for the first time. It has passed. Register now and HMRC sets your filing deadline three months from the date on its letter instead.
- 31 October 2026, 11:59pm. Paper return deadline.
- 30 December 2026, 11:59pm. File online by this date to ask for the tax to be collected through your PAYE code rather than paid in one go, if you qualify.
- 31 January 2027, 11:59pm. Online return, balancing payment, and first payment on account.
- 31 July 2027. Second payment on account.
The run up to 31 January 2027, month by month
Nothing here has to happen in January except the payment
Register if you never have. Recover the HMRC login, because the activation code comes by post. Pull statements, P60s and invoices together. 31 October is the paper return deadline.
Categorise. Decide what is allowable. Get the figure, so you have two months to fund it rather than one day.
File. Agree a payment plan now if you need one. 30 December is the deadline to file online and have the tax collected through your PAYE code.
Pay. If the three months above went as they should, that is all that is left.
Filing early does not mean paying early. The money is still due on 31 January whenever you file. Filing in October simply means knowing the figure in time to do something about it.
What should I do in October for my tax return?
This month is administrative, not technical. Register if you never have. Recover the HMRC login, which is the step that quietly costs people a fortnight because the activation code arrives by post. Pull together bank statements, invoices, P60s and P11Ds, interest certificates, dividend vouchers, rental statements and anything with Gift Aid or pension contributions on it.
If you are going to use an accountant, this is also the month to get agent authorisation moving, because that too goes through the post.
What should I do in November?
November is where the decisions are. What is allowable and what is not. Whether an expense is wholly and exclusively for the business or has a private element that has to come out. Whether last year’s payments on account already cover most of the bill.
The point of doing this in November is not virtue. It is that knowing the number in November gives you two months to fund it, and knowing it on 30 January gives you one day.
The January rush is a scheduling problem, not a tax problem. It is solved in October.
What should I do in December?
File. There is no advantage left in waiting, and the payment date does not move.
If the figure is more than you can comfortably pay, December is when to deal with that, not January. HMRC’s online Time to Pay route is open to people who owe less than £30,000 and meet the criteria, and you will need the reference for the tax, UK bank details authorised for a Direct Debit, and a picture of your income and spending.
30 December is the other date worth knowing. File online by then and you can ask for the tax to be collected through your PAYE code across the following year instead of paying it in one lump, if you have PAYE income and meet HMRC’s conditions.
What to do if you cannot pay by 31 January
Filing and paying are separate charges, so the first step never changes
A late return costs £100 immediately even with no tax to pay. That charge is avoidable even when the payment is not, so filing always comes first.
Set up a Time to Pay plan online, if you meet the criteria. You will need the tax reference, UK bank details authorised for a Direct Debit, and a picture of your income and spending.
Call HMRC before the deadline rather than after it. They will ask about income, outgoings, assets and debts, then tell you what to pay and when.
The mistake to avoid. Not filing because you cannot pay turns one charge into two. A late return and a late payment are charged separately and both run at once.
What is left to do in January?
If October, November and December went as above, January is one bank transfer.
If they did not, the order still matters. File the return even if you cannot pay, because the two charges are separate and a late return adds £100 immediately whether or not any tax is due. From three months late that becomes £10 a day capped at £900, then 5 per cent of the tax or £300 at six months, and the same again at twelve. The payment side runs at 5 per cent after 30 days, again at six months and again at twelve, with late payment interest at 7.75 per cent as at 10 October 2026.
“What a wonderful company! I didn’t actually need a tax advisor it turned out, but still got some excellent advice about what I needed to do for my self assessment. It’s not often people are kind and helpful when it comes to business.”
Can I still reduce my tax bill before 31 January?
A return reports a year that has closed, so most of the figure is fixed. Not all of it is, and these are the levers that are still live while the return is being prepared.
Pension contributions. A personal contribution extends your basic rate band by the gross amount, which is what actually moves a higher rate bill. The annual allowance is the ceiling and unused allowance from the three previous tax years can sometimes be carried forward. This only helps the 2025 to 2026 return if the contribution was made in that tax year, so check the date before assuming.
Gift Aid. Donations extend the basic rate band in the same way. There is also an election to carry a donation back to the previous tax year, and it has to be made on or before the date the return is filed, which means filing late can destroy the option entirely.
Payments on account. If this year’s income is genuinely lower, the July and January instalments can be reduced. If the real bill comes in higher, HMRC charges interest on the shortfall, currently 7.75 per cent as at 10 October 2026, so it is a forecast rather than a deferral.
Marriage Allowance. Where one partner has income below the personal allowance and the other is a basic rate taxpayer, part of the allowance can be transferred, and claims can usually be backdated. Small, but routinely missed on couples where one has stopped work.
The allowances versus the expenses. With trading income under about £1,000, or small rental income, the £1,000 trading allowance and £1,000 property allowance can beat claiming actual expenses. It is an either or choice per income stream, not both, and worth testing rather than assuming.
Which accounting basis. Cash basis or accruals changes which year income and costs land in, and for a small trade with slow paying customers the difference is not trivial.
Everything that can still change the figure has to be decided before the return is filed, not after.
What are the penalties for a late tax return and a late payment?
There are three separate regimes and they stack. Knowing which one you are in changes what you should do first.
Filing late. £100 the moment 11:59pm on 31 January passes, with no tax due and no excuse needed. From three months late, £10 a day for up to 90 days, capped at £900. At six months, 5 per cent of the tax or £300, whichever is greater. At twelve months, the same again. A return with no tax to pay still reaches £1,600 if it is a year late.
Paying late. Separate and additional. 5 per cent of the unpaid tax at 30 days, again at six months, again at twelve, with late payment interest running from day one at 7.75 per cent as at 10 October 2026. Interest is not a penalty and is not reduced for good behaviour.
Never telling HMRC at all. This is the one people have not heard of. Failing to notify chargeability is its own regime under Schedule 41 of the Finance Act 2008, and the penalty is a percentage of the tax rather than a flat fee. Non deliberate and disclosed unprompted more than twelve months after the tax was due runs from 10 to 30 per cent. Deliberate and concealed, prompted by HMRC getting there first, runs from 50 to 100 per cent. The difference between the top and the bottom of those ranges is almost entirely about whether you came forward or were found.
Reasonable excuse. HMRC can cancel a late filing penalty where something unusual and outside your control stopped you filing, and you then filed without unreasonable delay once it was resolved. Being busy is not one. Serious illness, a bereavement, a service failure at HMRC’s end or a fire or flood can be.
The three penalty regimes stack. Which one you are in decides whether filing or paying comes first.
What do I need before I start my Self Assessment return?
- Your Unique Taxpayer Reference and your Government Gateway login.
- P60, P45 and P11D for any employment.
- Business income and expenses for the year, with the bank account they ran through.
- Rental income and allowable property expenses, kept separate from the trade.
- Dividend vouchers, interest certificates, and anything disposed of that might produce a gain.
- Pension contributions and Gift Aid donations, both of which extend the basic rate band.
- Student loan plan type, if you have one.
If one of those is a rental property you have not declared before, do not put it on this return without reading what the earlier years involve first. Our guide to interest on undeclared rental income explains why.
“I have used Total Books to do my self assessment tax return for 2 years now and they are wonderful. Very patient with my bookkeeping, always explain every step and I file with the confidence I haven’t over paid my taxes but also that I am being completely transparent.”
Know the number before Christmas
We file as your HMRC agent for a fixed fee agreed up front, and we tell you what you owe with months left to plan for it.
What should I do if HMRC has written to me about my tax return?
A letter changes the arithmetic, so deal with it differently from a deadline you are simply approaching.
The important distinction is prompted against unprompted. A disclosure you make before HMRC contacts you sits at the bottom of the penalty range. One made after they write sits at the top. In the failure to notify regime that is the difference between 10 per cent and 20 per cent for non deliberate behaviour, and between 20 and 35 per cent where it was deliberate. The letter does not change the tax. It changes the penalty band you are negotiating inside.
So if something has arrived, the order is: read what it actually asks for, work out which years it covers, and get advice before replying. Do not ignore it, and do not send partial figures to buy time, because an incomplete response is treated worse than a late complete one.
What happens on the first call with an accountant?
Fifteen minutes, no paperwork needed, and nothing is signed at the end of it.
We ask what income you had, whether you have filed before, whether anything has come from HMRC, and how close you are to having records. From that we can tell you whether the January deadline is comfortable or tight, what the return involves, and what the fixed fee is.
If you are reading this in October or November, the honest answer is usually that you have plenty of time and the whole thing is smaller than it feels. That is worth hearing from someone who is not trying to sell you a panic.
Find out now whether your January is going to be straightforward or difficult. Fifteen minutes, a fixed fee, and no chasing afterwards.
Or call 029 2002 6505
By the time the mornings look like this, the only thing still movable is how organised you are.
We take on Self Assessment clients across Cardiff, Newport and Bristol through to the end of December, and we would rather see you in October.
The 31 January deadline: common questions
What exactly is due on 31 January 2027?
Three things on one date: the online return for the 2025 to 2026 tax year, the balancing payment for that year, and the first payment on account for 2026 to 2027. The deadline is 11:59pm. The second payment on account follows on 31 July 2027.
I have left it until January. Is it too late to get help?
Not impossible, but it is the hardest month to get taken on, because the work of chasing records and getting agent authorisation in place does not compress. If you are reading this in October or November you have the easy version of the problem.
Does filing early mean paying early?
No. You can file in October and the money is still not due until 31 January. Filing early just means you know the figure in time to do something about it, and it is the single cheapest thing you can do about a January bill.
What is the 30 December date for?
If you file online by 11:59pm on 30 December 2026, you can ask for the tax to be collected through your PAYE code during the following year instead of paying it in one go. It only works if you have PAYE income and you meet HMRC’s conditions, and it is the one deadline almost nobody knows about.
What happens if I file late?
£100 the moment the deadline passes, with no tax due and no excuse needed. From three months late it becomes £10 a day capped at £900. At six months it is 5 per cent of the tax or £300, whichever is greater, and the same again at twelve months. Paying late is charged separately.
Can I still register if I never have?
Yes, and you should do it today. The registration deadline for the 2025 to 2026 year was 5 October 2026. Having missed it, HMRC will set your filing deadline at three months from the date on its letter, and the penalty exposure is for failing to notify rather than for filing late.
Can I still reduce my tax bill after the tax year has ended?
Some of it. A pension contribution or Gift Aid donation only helps the year it was actually made in, but the Gift Aid carry back election and the choice between the trading or property allowance and actual expenses are both still live while the return is being prepared. The carry back election has to be made on or before the date you file, so filing late removes it.
What is a reasonable excuse for filing late?
Something unusual and outside your control that stopped you filing, where you then filed without unreasonable delay once it was resolved. Serious illness, a bereavement, a fire or flood, or a service failure at HMRC’s end can qualify. Being busy, or finding the system confusing, does not.
HMRC has written to me about a return. What should I do first?
Read what it actually asks for and which years it covers, then get advice before replying. The difference between a disclosure you make before HMRC contacts you and one made afterwards is the difference between the bottom and the top of the penalty range, so a considered reply beats a fast partial one.
Is there any benefit to filing in October rather than January?
Three. You know the figure with three months to fund it rather than one day. If you are due a refund, which is common under CIS, it arrives sooner. And you still have the 30 December option to have the tax collected through your PAYE code rather than paid in a lump.
Does filing early mean HMRC takes the money early?
No. The payment date is 31 January whenever you file. Filing in October and paying on 31 January is the normal, sensible pattern and there is no downside to it.
What if I cannot pay and cannot face dealing with it?
File the return anyway, because the filing penalty and the payment penalty are separate and the first one is avoidable even when the second is not. Then deal with the payment through Time to Pay, which is open online to people who owe less than £30,000 and meet the criteria. Doing nothing is the only option that makes both charges worse.
Related reading
If the thought of January is already sitting badly, fifteen minutes now will tell you what your return involves, what it will cost and roughly what you will owe. All three answers are better known in October than on 30 January.
Deadlines, penalties and interest stated as published on GOV.UK at 10 October 2026. General information, not advice on your own position.
