Every January, I hear the same thing from a small number of clients: "I didn't realise I'd owe that much." Usually said on 29 or 30 January, with the Self Assessment deadline two days away.
It's not that they've done anything wrong. It's that the bill lands bigger than they expected, at the worst possible time — right after Christmas, when the bank balance has already taken a hit from presents, travel, and the general cost of the festive season.
For the tax year ending 5 April 2026, your Self Assessment return — and the tax you owe for it — is due on the same day: 31 January 2027. That's not a coincidence worth glossing over. The filing deadline and the payment deadline are the same date. Get the number wrong, or leave it too late to find out what it is, and you're paying HMRC and buying Christmas presents out of the same bank balance in the same few weeks.
The bill is usually bigger than people expect — here's why
The part that catches people out most often isn't the tax on the year itself. It's payments on account.
If your tax bill is above a certain threshold, HMRC doesn't just ask you to pay what you owe for the year that's just ended. It asks you to pay half of next year's estimated bill as well, in advance, based on this year's figures. So the amount due on 31 January isn't one number — it's last year's balancing payment plus the first instalment toward the year that's still running. For a lot of people, that roughly doubles the number they were picturing.
I see this pattern often: someone works out roughly what they think they owe, feels reasonably comfortable with that figure, and doesn't check again until the return is actually done. Then the payment-on-account figure appears, and the number on screen is a lot higher than the number in their head. That's a stressful thing to discover with a few days' notice — and it's avoidable with none of the stress, simply by knowing the full number early enough to plan around it.
Christmas spending and your tax bill are competing for the same money
This is the bit that doesn't get said out loud enough: if you don't know your tax position before December, you're making Christmas spending decisions without knowing what's coming for it in January.
Christmas costs add up quietly — a bit here for gifts, a bit there for travel, meals out, the things you hadn't quite budgeted for. None of it feels reckless in the moment. It's only when you add it all up against a tax bill you haven't calculated yet that the squeeze becomes real. And it's a bad kind of surprise to discover in January that the amount you'd happily have spent in December actually needed to go to HMRC.
Getting ahead of your Self Assessment return now — in November or early December — tells you exactly what you can afford to spend over Christmas, because you know what's already accounted for. That's not about being cautious for its own sake. It's about being able to enjoy the festive period properly, rather than spending it with a vague, nagging sense that there's a bill coming you haven't looked at yet.
What being organised actually buys you
As a Chartered Accountant, the thing I'd want any client to take from this is simple: January is already a rushed month. Everyone's return lands at once, everyone's deadline is the same day, and even a good accountant working flat out is managing a queue. Your accountant can usually still hit the deadline for you. What they can't do is give you back the weight of not knowing, for weeks, whether the number is going to be manageable or not.
Getting your return done early doesn't change what you owe. It changes when you find out — and that's the part that actually matters to how your December and January feel. Clients who get this sorted well before the deadline aren't dealing with it on New Year's Day with a hangover and a spreadsheet. They've had the number in hand since before Christmas, planned around it, and can get on with January — gloomy as it already is — without a tax deadline hanging over it as well.
Where Making Tax Digital fits in
If you're self-employed or a landlord above the relevant income threshold, Making Tax Digital for Income Tax (MTD ITSA) is changing how often you need to report to HMRC — moving from one annual return to quarterly updates through compatible software. That's a separate change to the January deadline itself, but the underlying point is the same: the businesses and individuals who keep their records current through the year are never caught out by either requirement. If your records are only ever brought up to date once a year, in a rush, every deadline — quarterly or annual — feels like this one.
This is part of why we build Making Tax Digital set-up into how we work with clients from the start, rather than treating it as a separate project to deal with later. See our Accounts & Year End package, which covers Self Assessment and MTD for Income Tax alongside your other filings, tracked against your actual deadlines through the year rather than left until January.
What to do now, before Christmas
If you haven't started your return for the year ending 5 April 2026, here's the honest, practical version of what to do:
- Get your records together now. Income, expenses, anything you'll need. The sooner this is done, the sooner you have a real number, not a guess.
- Ask specifically about payments on account. If this is new to you, or your income has grown this year, don't assume last year's bill is a guide to this year's. Ask the question directly.
- Treat the number as a planning input, not a January problem. Once you know what's due on 31 January, you know what you can actually spend over Christmas without putting yourself under pressure in the new year.
- Don't wait for the deadline to ask for help. The earlier a return comes to us, the calmer the process is — for you and for us.
When early isn't realistic
I want to be honest about where this advice has limits. If you're newly self-employed and genuinely don't have your records together yet, or your income for the year is still unsettled because of work that's only just landed, you can't produce a meaningful number in November — and forcing one would just give you a false sense of certainty. In that situation, the right move isn't to panic about being "behind." It's to get your records into some kind of order as soon as you reasonably can, and have an honest conversation early about what the range of outcomes might look like, even if the exact figure isn't there yet. An early rough estimate is still far more useful than no estimate until 30 January.
The point of all this
None of this is about getting a better tax outcome. Doing your return in November instead of January doesn't change what you owe HMRC. What it changes is whether you spend Christmas with a clear picture of your finances, or with a number hanging over you that you haven't looked at yet.
If you'd like help getting your Self Assessment return sorted well ahead of the 31 January deadline — payments on account included — get in touch. The sooner we start, the more relaxed the whole thing is for everyone involved.