The short version
- Registering for Self Assessment is a one-off step, done by 5 October following the end of the tax year in which self-employed trading actually started - not something repeated every year.
- A UTR (Unique Taxpayer Reference) arrives by post a couple of weeks after registering, and is the number used on every return afterwards.
- The online filing deadline is 31 January, the following year, for the tax year that just ended - the single date most of the process actually revolves around.
- "Payment on account" is the part that catches most first-time filers by surprise - two advance instalments toward next year's bill, paid alongside the current year's tax, which is why a first Self Assessment bill often looks larger than expected.
- Filing late costs a fixed £100 penalty from day one, even with nothing owed, rising the longer it is left outstanding.
Step one: registering, and when it actually has to happen
Checked 26 September 2026: registering for Self Assessment is a one-off step, not an annual one - done once, the first time self-employed trading genuinely starts, by 5 October following the end of the tax year that trading began in. A shop that started selling in the 2026/27 tax year, for example, needs to register by 5 October 2027 at the latest - well after trading itself began, giving genuine time to get the paperwork side sorted rather than needing it done from day one.
Step two: the UTR, and why it matters
Once registered, HMRC posts a Unique Taxpayer Reference - a ten-digit number that identifies a specific taxpayer's own Self Assessment record, needed on every return and most correspondence with HMRC afterwards. It can take up to 21 working days to arrive after registering, by post rather than instantly online, which is itself a reason not to leave registering until the very last moment before a filing deadline actually approaches.
Step three: the deadline the whole process revolves around
The online Self Assessment filing deadline is 31 January, following the end of the tax year being reported - the return for the tax year ending 5 April 2027, for example, is due by 31 January 2028. This is the one date genuinely worth building a shop's own calendar around: the return itself, and any tax owed for that year, are both due on the same day.
The part that genuinely surprises most first-time filers
"Payment on account" is the single concept most responsible for a first Self Assessment bill looking larger than a straightforward calculation of the past year's tax would suggest. Alongside the tax actually owed for the year just finished, HMRC often also collects two advance instalments toward the following year's bill - the first due on the same 31 January date, the second on 31 July - each based on half of the previous year's total. A seller expecting to pay exactly last year's tax bill and finding a noticeably larger total due is very often looking at this year's bill plus a head start on next year's, not an error or an unexpected extra charge.
This is not extra tax being charged twice - it is the same total liability spread earlier, with the following January's actual bill reduced by whatever was already paid in advance the previous July. Knowing this in advance is mainly about not being caught out by the total the first time it happens.
Who genuinely doesn't have to make these advance payments
Checked 27 September 2026, directly against GOV.UK's own guidance on understanding a Self Assessment bill: payments on account are not universal, and two specific exceptions cover a genuine share of smaller sellers. They do not apply if the tax owed for the previous year was less than £1,000, or if more than 80% of the tax owed was already collected outside Self Assessment - through a main job's tax code, for instance. A seller running a genuinely small side shop alongside a main PAYE job, with a modest Self Assessment bill under that £1,000 threshold, may never actually encounter payments on account at all, which is worth knowing plainly rather than assumed to be an unavoidable part of the process for absolutely everyone who registers.
What to do if the coming year genuinely looks smaller than the last one
Because each payment on account is calculated from half of the previous year's tax bill, a shop having a genuinely quieter year than the one just finished - a supplier lost, hours cut back, a deliberate slowdown - can end up prepaying more than the year actually turns out to owe. GOV.UK's own guidance sets out a direct remedy: a seller expecting the current year's tax to come in lower can ask HMRC to reduce the payments on account, either online through the same Self Assessment account used to file, or by post using form SA303, providing an honest estimate of the year's actual expected tax. The genuine catch worth knowing before doing this: if the real bill later comes in higher than the reduced estimate, HMRC charges interest on the shortfall from the original due date, so this is worth doing from a genuine, considered estimate of the year ahead, not simply a hopeful guess made to reduce what is due in January.
What happens if the deadline is missed
Filing late triggers a fixed £100 penalty from the first day past the deadline, regardless of whether any tax is actually owed - a return with nothing due still incurs it if filed late. Left longer, further penalties accumulate: a daily charge after three months, up to a further £900, with additional penalties at six and twelve months on top of that. Separately, tax paid late accrues interest and its own penalties at 30 days, six months and twelve months. The one genuinely reliable way to avoid all of it is filing and paying by 31 January, which is a single, well-publicised date rather than a moving target.
What actually counts if a deadline genuinely was missed for a real reason
A late penalty is not automatically the final word if there was a genuine, specific reason the deadline was missed. GOV.UK's own guidance on reasonable excuses lists concrete examples HMRC will actually accept: an unexpected hospital stay, a serious illness, the death of a close relative shortly before the deadline, a computer or software failure while preparing the return, HMRC's own online services being down, or postal delays that could not have been predicted. It is worth knowing, just as specifically, what does not count: a bounced cheque or failed payment because funds simply were not there, finding the online system too difficult, not receiving a reminder from HMRC, or a genuine mistake on the return itself. The return or payment still has to be made as soon as actually possible once the excuse no longer applies - a reasonable excuse delays the obligation, it does not remove it.
A worked example, dates included
A shop starting to trade in July 2026, inside the 2026/27 tax year: registration is due by 5 October 2027 at the very latest, though registering sooner - once it is clear the trading is genuine and ongoing - avoids the UTR arriving later than convenient. The first return, covering the full 2026/27 tax year, is due online by 31 January 2028, with any tax owed, and potentially a first payment on account toward 2027/28, due the same day.
The earlier deadline that only matters for a paper return, and a genuinely useful option hiding inside it
GOV.UK's own page on Self Assessment deadlines is worth checking directly for a detail most online-filing sellers never need to think about: a paper return has its own, considerably earlier deadline of 31 October, three full months ahead of the 31 January online date - filing on paper after that date triggers the same late penalty, even with three months still notionally left before the online deadline most sellers actually use. The same page also confirms a genuinely useful option worth knowing about for a seller who also has a PAYE job: submitting an online return by 30 December, rather than waiting until 31 January, opens the possibility of having tax owed collected through a future tax code adjustment instead of paid as a single lump sum - worth knowing about specifically for a seller who would genuinely prefer smaller amounts spread through the following year's payslip over one larger payment in January.
What actually needs doing, reduced to the essentials
Stripped of the jargon: register once, by 5 October after trading starts; keep an accurate record of sales and genuine business expenses throughout the year; file and pay by the following 31 January; and budget for payment on account as part of the total due, not a surprise on top of it. Everything else is detail sitting underneath these four actual steps.
Where Sellhelm actually fits
Sellhelm's own Finances page keeps sales and costs organised under the same headings a Self Assessment return actually uses, ready to hand to an accountant or enter directly - the calculation itself, and the registration and filing steps above, stay a seller's own responsibility, or their accountant's, rather than something any tool files on their behalf.
Questions people ask
When do I need to register for Self Assessment as an eBay seller?
By 5 October following the end of the tax year in which self-employed trading started - a one-off registration, not an annual one.
What is a UTR and how do I get one?
A Unique Taxpayer Reference, a ten-digit number HMRC posts out within a couple of weeks of registering for Self Assessment - needed on every return afterwards.
Why is my first Self Assessment bill bigger than expected?
Very likely "payment on account" - two advance instalments toward next year’s tax bill, paid alongside the current year’s tax, which makes a first bill look larger than a simple calculation of that year’s tax alone.
What happens if I file my Self Assessment return late?
A fixed £100 penalty applies from day one, even with no tax owed, rising further after three, six and twelve months - alongside separate interest and penalties if any tax owed is also paid late.