Your First Self Assessment / Lesson 3 of 7
How side income is taxed on top of a salary
You earned £2,000 on the side and kept about £1,200. Where did the rest go?
The short answer
- ✓ Side income stacks on top of your salary and starts at your marginal rate
- ✓ Your personal allowance is already used by the job
- ✓ Class 4 NI applies to side profit even though you pay Class 1 at work
- ✓ Filing online by 30 December can let HMRC collect the bill through your code
Side income stacks on top
Your income is taxed in slices, and your salary fills the lower slices first. Side income therefore starts wherever your salary finished, which is why the effective rate on it feels high.
Salary £46,000
Personal allowance already used all of it
Basic rate band already used most of it
Side income £2,000 lands partly at 20%
and partly at 40%, plus Class 4 NI.
Nothing is tax free, because the allowance
was consumed by the salary before the side
income was even considered.Someone whose only income was that £2,000 would pay nothing at all on it. Same money, completely different outcome, purely because of what sits underneath it.
National Insurance stacks differently
You pay Class 1 NI through payroll on your salary, and Class 4 on your self-employed profit as a separate charge. They do not merge, and your employment does not exempt the side income.
Class 2 is worth checking rather than assuming: if your employment already gives you a qualifying year for the State Pension, paying Class 2 voluntarily on a small side profit adds nothing.
Why the tax bill arrives all at once
PAYE collects tax on your salary as you earn it. Nothing collects tax on your side income during the year, so the whole liability lands on 31 January. People spend the side income as it arrives and then meet the bill months later, which is the most common cash-flow failure for first-time filers.
The bit HMRC does not spell out
Where the amount is modest, HMRC can collect it through your tax code the following year instead of demanding a lump sum, provided you file online by 30 December rather than 31 January. Most people file in the last week of January and lose the option without ever knowing it existed.
Filing a month early can therefore convert a January bill into twelve smaller monthly deductions. That is worth knowing before you leave it to the deadline.
Common mistakes
- Expecting the personal allowance to apply again. Your salary already used it.
- Not setting money aside. Nothing is collected during the year.
- Filing on 31 January. The code-collection option closes on 30 December.
- Assuming employment covers your NI. Class 4 applies to the profit separately.
Salary plus side income
Where the tax actually comes from
National Insurance, student loan and pension contributions are left out here on purpose, so you can see income tax on its own. The calculator below includes them.
Try it on your own numbers
This is the same calculator as the full tool page, using 2026/27 rates.
Your side hustle
Tax on a second income (freelancing, selling, gig work) stacked on top of your main job. Updates as you type.
Includes the £1,000 trading allowance. The "best of both" option uses whichever of the allowance or your real expenses saves you more - you cannot claim both.
Tax on your side hustle
on of side income - you keep
- Deduction used
- Taxable side profit
- Income Tax on it
- Class 4 NI on it
- You keep
kept of side income
tax on next £100
Register for Self Assessment
Your side income is over £1,000, so you must register with HMRC and file a tax return.
Estimate only. Assumes Class 4 NI; small profits may also owe voluntary Class 2.
Tax as your side income grows
How your take-home and tax change as the side hustle scales, on a salary.
Compare saved scenarios
| Scenario | Side income | Tax + NI | You keep | |
|---|---|---|---|---|
Key takeaways
- ✓ Side income stacks on top of your salary and starts at your marginal rate
- ✓ Your personal allowance is already used by the job
- ✓ Class 4 NI applies to side profit even though you pay Class 1 at work
- ✓ Filing online by 30 December can let HMRC collect the bill through your code
Check you have got it
3 quick questions. No score is kept, and you can change your mind.
1. Why is your side income taxed from the first pound?
2. What happens if you file online by 30 December instead of 31 January?
3. You pay Class 1 NI on your salary. Does that cover your freelance profit?
Sources
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