The UK collects income tax in two main ways. PAYE takes tax automatically from your pay before you see it. Self Assessment asks you to calculate and pay your own tax once a year. Most people only ever meet PAYE, but the moment you pick up freelance work, rental income, large investment returns, or simply earn enough, Self Assessment can enter the picture, sometimes alongside PAYE rather than instead of it. This guide explains how each system works and how they fit together.
PAYE: tax collected at source
PAYE stands for Pay As You Earn. If you are an employee or receive a workplace or private pension, your employer or pension provider deducts income tax and National Insurance before paying you, and sends it to HMRC on your behalf. You receive the net amount, and in most cases you never fill in a form.
The mechanism rests on three things:
- Your tax code, which tells the payer how much tax-free allowance to apply.
- A cumulative calculation, where each pay period looks at your total pay and total allowance for the year so far, so deductions self-correct over time.
- HMRC reconciliation, where after the tax year HMRC checks whether the right amount was collected and issues a refund or a bill if not.
The headline advantage of PAYE is that it is automatic and spread evenly across the year. You pay as you go, in small regular amounts, and for a simple employment situation it gets very close to correct on its own. Our take-home pay calculator reproduces what PAYE should be deducting from a given salary.
Where PAYE alone falls short
PAYE is excellent for steady employment income, but it only knows what the payer tells it. It cannot see income that does not pass through that payroll: freelance earnings, profits from a side business, rental income, large amounts of savings interest or dividends, or capital gains. It also struggles to handle higher-rate relief on some pension and charitable contributions automatically. When income or reliefs sit outside what PAYE can see, Self Assessment fills the gap.
Self Assessment: you calculate and pay
Self Assessment is the system for reporting income that is not fully taxed at source. Instead of an employer doing the work, you (or your accountant) complete a tax return each year that totals up all your taxable income, applies allowances and reliefs, and arrives at a tax figure. You then pay HMRC directly.
The defining features:
- You declare the income yourself, across all sources, on one return.
- You pay in lump sums, typically after the tax year ends, rather than continuously.
- Deadlines and penalties apply, with fixed dates for filing and paying, and charges for missing them.
Self Assessment is how the self-employed, landlords, and people with substantial untaxed investment income settle their tax. Our Self Assessment calculator estimates the bill from your income figures, and the self-employed also have National Insurance to account for through the same return, which our self-employed National Insurance calculator covers.
Who needs to file a Self Assessment return
You typically need to file when you have income that PAYE cannot tax for you, or circumstances that PAYE cannot handle. Common triggers include:
- Being self-employed or a sole trader above a small trading threshold.
- Being a partner in a business partnership.
- Receiving rental income above the level PAYE can absorb.
- Having significant untaxed savings, investment, or dividend income.
- Reaching a high income where the personal allowance is withdrawn or the High Income Child Benefit Charge applies.
- Making capital gains above the annual exempt amount.
- Wanting to claim certain reliefs that do not flow through PAYE automatically.
If none of these apply and you are a straightforward employee, you usually do not need to file at all. PAYE handles everything.
The crucial point: the two systems work together
The most common misunderstanding is treating PAYE and Self Assessment as either/or. For many people they run side by side.
Consider an employee with a freelance sideline. Their salary is taxed through PAYE as normal. Their freelance profit is not taxed at source, so it goes on a Self Assessment return. On that return, HMRC does not ignore the salary, it includes the salary and the tax already paid through PAYE, then works out the total tax due on everything combined, and the Self Assessment bill is the extra owed on top of what PAYE already collected.
This stacking matters because the freelance income sits on top of the salary for rate purposes. If the salary already fills the basic rate band, the freelance profit may be taxed entirely at the higher rate, even though in isolation it looks like a small amount. Our income tax calculator shows how additional income stacks on top of existing earnings.
A worked example with round numbers
Suppose, using simple illustrative figures:
- An employee earns £45,000 from a job, all taxed through PAYE.
- They also make £10,000 profit from freelance work.
- The basic rate is 20% up to £50,000 of total income, and 40% above.
Their combined income is £55,000. The freelance £10,000 sits on top of the £45,000 salary, so:
- £5,000 of it falls in the remaining basic rate room (£45,000 to £50,000), taxed at 20%, which is £1,000.
- £5,000 of it falls into the higher rate band (above £50,000), taxed at 40%, which is £2,000.
So the Self Assessment income tax on the freelance profit is £3,000, plus any self-employed National Insurance due. The job’s tax was already settled through PAYE and is not paid again, it simply set the starting point that pushed half the freelance income into the higher band.
Payments on account: paying ahead
One feature of Self Assessment surprises new filers: payments on account. Once your Self Assessment bill passes a certain size, HMRC asks you to pay toward next year’s tax in advance, usually in two instalments, on the assumption your income will be similar. So in your first year you can face the previous year’s bill and an advance payment toward the next at the same time, which feels like paying twice. It is not a penalty, it is the system shifting you from paying entirely in arrears toward paying closer to as-you-go, the way PAYE already works. If your income falls, you can apply to reduce the payments on account.
Key differences at a glance
The two systems differ along a few clear lines:
- Who does the maths: PAYE, your payer; Self Assessment, you.
- When you pay: PAYE, continuously through the year; Self Assessment, in lump sums, often with advance payments on account.
- What it covers: PAYE, income passing through a payroll or pension; Self Assessment, everything else and the reconciliation of the total.
- Effort: PAYE, none for a simple case; Self Assessment, a return each year with hard deadlines.
Frequently asked questions
If I am employed, do I ever need to do Self Assessment?
Often not. A straightforward employee whose only income is salary is handled entirely by PAYE. You generally only file if you have untaxed income (freelance work, rent, large investment returns), reach a high-income threshold, or need to claim reliefs PAYE cannot apply automatically.
Does Self Assessment mean I pay tax twice on my salary?
No. Your Self Assessment return includes your salary and the tax already deducted through PAYE. HMRC works out the total due on all income, then credits what PAYE already collected. You only pay the additional tax on income that was not taxed at source.
Why does HMRC want me to pay next year’s tax in advance?
That is the payments-on-account system. Once your Self Assessment bill is large enough, HMRC asks for advance instalments toward the following year, moving you closer to paying as you earn. It is not an extra tax, and you can ask to reduce the payments if you expect your income to drop.
How is my freelance income taxed if I already have a salary?
It stacks on top of your salary. Your salary uses up the lower bands first, so freelance profit is taxed at whatever band it lands in once added to your earnings. A modest amount of freelance income can therefore be taxed at the higher rate if your salary has already filled the basic rate band.