The short version
- National Insurance paid through employment does not cover self-employed profit made on the side - the two are entirely separate charges, assessed against separate income, both genuinely due.
- Class 4 National Insurance applies to self-employment profit above £12,570, at 6% up to £50,270 and 2% above it - on top of, not instead of, Class 1 already paid through a job.
- Class 2 National Insurance no longer has to be paid as a separate weekly charge since April 2024 - profit at or above the Small Profits Threshold (£7,105) is automatically credited toward the State Pension with nothing to actually pay.
- Someone paying both Class 1 and Class 4 in the same tax year can, in some circumstances, genuinely overpay - and unlike Class 1 deferment for a second job, Class 4 itself cannot be deferred in advance, only reclaimed afterwards.
- A specific HMRC form exists for exactly this refund, worth knowing about directly rather than assumed not to apply.
The assumption worth correcting first
"I already pay National Insurance through my job" is true, and it is also not the same thing as having covered any tax due on separate, genuine self-employment profit made on the side. Employment income and self-employment profit are assessed separately for National Insurance purposes - Class 1 through PAYE on the job, and a genuinely distinct charge on self-employed profit - and paying one does not exempt the other. This is easy to miss precisely because both are called "National Insurance," which reads, reasonably enough, like a single pot rather than two separate ones.
Class 4: the one that actually applies to profit
Checked 28 September 2026: Class 4 National Insurance is charged on self-employment profit above £12,570 a year - 6% on profit between £12,570 and £50,270, and 2% on anything above that. This applies to the profit itself, regardless of whatever else is happening on the employment side of a person's income - a side income of genuine profit above that threshold owes Class 4 on top of whatever Class 1 is already being paid through a main job, not instead of it.
Class 2: simplified since 2024, but not entirely gone
Class 2 National Insurance no longer requires an actual weekly payment for most self-employed people - since April 2024, profit at or above the Small Profits Threshold, £7,105, is treated as though Class 2 has already been paid, crediting that year toward the State Pension automatically with nothing to actually hand over. Below that threshold, paying Class 2 becomes voluntary rather than compulsory - a modest weekly amount available specifically to make sure a lower-profit year still counts toward the State Pension record, rather than a bill that arrives regardless.
For someone already in full-time employment with a modest side income underneath the Small Profits Threshold, this voluntary payment rarely matters in practice - their main job's own Class 1 contributions are very likely already building a full State Pension qualifying year on their behalf, independent of what a small side profit does or does not add on top.
Why Class 1 deferment exists for a second job, and why it doesn't help here
It is worth knowing that a real deferment mechanism does exist for National Insurance, since it is a common point of confusion for anyone juggling more than one source of income - but it applies to a genuinely different situation than the one this post is about. The government's own guidance on deferring National Insurance confirms this is specifically for an employee paying Class 1 with more than one employer, who can apply to pay a reduced 2% rate on one job's earnings rather than the full rate on both, to avoid overpaying across the combined total. The same guidance is direct on the point that actually matters for a self-employed side income: "if you're self-employed, you cannot defer Class 4 contributions." An eBay shop run alongside a main job is not a second employment in this specific sense, and there is no equivalent way to apply in advance to pay Class 4 at a reduced rate because of an already-busy main job's own contributions.
What actually exists instead: a genuine refund, claimed after the fact
Where deferment isn't available, the same government guidance points to the real alternative: someone who is self-employed "may be able to claim a refund for previous tax years" if their combined Class 1 and Class 4 contributions across a tax year turn out to have exceeded what was actually owed. HMRC's own published form CA5610 exists specifically for this - a genuine, named route for reclaiming overpaid Class 4 contributions, available either as an online application or a printable postal form, rather than an obscure, undocumented process a seller would have to argue for from scratch. This is worth knowing about directly rather than assumed not to apply: a seller who has never checked whether their own combined employed-and-self-employed contributions for a specific year came out higher than they should have has no way of knowing, one way or the other, without actually running the numbers or asking HMRC to check.
Roughly when this genuinely becomes worth checking, rather than assumed for everyone
This is not a situation that applies to every employed-and-self-employed combination - it becomes a genuine possibility specifically where a main job's own earnings already sit reasonably high, close to or above the upper threshold where the lower 2% Class 1 rate applies, and a meaningful self-employed profit is then taxed at the full 6% Class 4 rate on top of it. For someone on a modest main-job salary with a small side profit, the combined total is very unlikely to exceed what is genuinely owed, and chasing a refund that doesn't actually exist wastes time better spent elsewhere. For someone with a higher-earning main job and a genuinely substantial side profit, actually running the sums - or simply asking HMRC directly whether an overpayment for a specific tax year looks likely - is worth doing rather than assumed away in either direction.
What the Class 2 credit is actually building toward, in concrete terms
It is worth knowing what a genuine State Pension qualifying year is actually worth, rather than treating the Class 2 credit as an abstract technicality. The government's own guidance on the new State Pension confirms a minimum of 10 qualifying years is needed to receive any new State Pension at all, and its own page on what you'll actually get confirms the full rate - currently £241.30 a week - requires 35 qualifying years for anyone whose National Insurance record started after April 2016. This is the real, concrete thing a Class 2 credit is quietly contributing toward every year it applies - not an abstract compliance box, but a specific, measurable step toward a specific weekly income later in life, which is worth knowing plainly given how easy it is to think of National Insurance purely as a cost with nothing tangible attached to it.
Why it's worth checking the actual record directly, rather than assuming it's fine
Given how directly a qualifying year translates into real State Pension income, it is worth using HMRC's own State Pension forecast tool directly to check a genuine, current record rather than assuming years are accumulating correctly in the background. This is worth doing specifically for anyone whose employment history includes genuine gaps, part-time periods, or years of self-employment below the Small Profits Threshold where no automatic credit applied before the 2024 change described above - a real, checkable gap in the record is far better discovered years in advance, while voluntary contributions can still fill it, than discovered for the first time on the edge of retirement when the options for correcting it have narrowed considerably.
A worked example
Someone employed full time, paying Class 1 through PAYE as normal, with a genuine £18,000 profit from an eBay shop run alongside it: Class 4 applies to the profit above £12,570 - so roughly £5,430 of it, at 6% - a real, additional amount due through Self Assessment, on top of whatever Class 1 their employer has already deducted. Nothing about the employment income reduces or offsets this; the two are simply separate calculations against separate income, both genuinely payable. A second seller, on a considerably higher main-job salary already close to the upper National Insurance threshold, running a similarly-sized side profit taxed at the full Class 4 rate on top: a genuinely more plausible candidate for checking whether the combined total has tipped over into a real overpayment worth reclaiming through form CA5610.
Why there are two classes at all
The two classes are doing genuinely different jobs, which is worth knowing mainly so the distinction stops feeling like arbitrary extra complexity: Class 2 is specifically about building a qualifying year toward the State Pension, a fairly small, flat amount regardless of how much profit was actually made; Class 4 is a genuine tax on the level of profit itself, rising with it in the same way Income Tax does. A side income can, in principle, owe Class 4 without ever touching Class 2 at all - or need voluntary Class 2 without owing any Class 4 - because the two are answering different questions about the same profit figure.
How this actually gets paid, in practice
Class 4 is not a separate bill arriving on its own schedule - it is calculated and paid as part of the same Self Assessment return and 31 January deadline that reports the profit itself, alongside Income Tax on that same profit. There is no separate registration step or separate payment date to track beyond the Self Assessment process already worth understanding on its own terms.
Where this stays worth an accountant's own check
Sellhelm's own Finances page keeps the real profit figure this calculation is actually based on, but the specific National Insurance position for a given tax year - especially for anyone with other income, other self-employment, or a genuinely unusual year - stays exactly the kind of thing worth a specific check against current rates rather than assumed to be a fixed, simple sum every year.
Questions people ask
Do I pay National Insurance on eBay income if I already have a job?
Yes, if the self-employed profit is above £12,570 - Class 4 National Insurance applies to it separately, regardless of the National Insurance already paid through employment.
Do I still need to pay Class 2 National Insurance?
Not as a compulsory weekly payment for most people since April 2024 - profit at or above £7,105 is automatically credited. Below that, it becomes a voluntary payment for anyone who wants the year to count toward their State Pension.
What are the current Class 4 National Insurance rates?
6% on self-employment profit between £12,570 and £50,270, and 2% on profit above £50,270.
When and how is Class 4 National Insurance actually paid?
As part of the same Self Assessment return and 31 January deadline that reports the profit and Income Tax on it - not a separate bill on its own schedule.