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| You must enter your full P60 details onto your Self-Assessment, so HMRC knows how much tax your employer has already paid. |
How Do You Do a Self-Assessment If You Are Also Employed Full-Time?
With the rise of side hustles, freelance gig work, and property investing, more and more people in the UK are balancing a traditional 9-to-5 job with extra streams of income.
If you are employed full-time, your employer automatically handles your standard income tax and National Insurance through the PAYE (Pay As You Earn) system. However, PAYE cannot automatically calculate tax on your extra earnings. If you make over £1,000 from a side hustle, you must complete a Self-Assessment tax return. Here is exactly how to declare both incomes without making a mistake.
1. Get Your P60 Form Ready First
The biggest mistake people make is assuming they only need to declare their side-hustle money on their Self-Assessment. HMRC needs to look at your total overall income for the tax year.
Before you start your tax return, you must ask your employer for your P60 form (or your final P45 if you left the job during that tax year. Your P60 lists:
Your total gross pay from your full-time job.
The exact amount of tax your employer already deducted and sent to HMRC.
You will need to type these exact numbers into your tax return, so HMRC does not charge you twice for your employment tax.
If you aren’t entirely sure whether your extra income crosses the legal limit, check our guide on how do you know if you need to register for Self Assessment before getting started.
2. Fill Out the "Employment" Section
When you log into the official GOV.UK Self-Assessment portal, the online software will ask you a series of "Tailor Your Return" questions.
Look for the question: "Were you an employee (or director) of a company?"
Click Yes.
This action opens up a specific "Employment" section. You will be prompted to type in your employer’s name, your Employer PAYE Reference number (found on your P60), your total pay before tax, and the tax already taken off. Simply copy these figures straight from your P60 form into the matching boxes.
3. Fill Out Your Side Hustle Section
Next, you will navigate to the section that matches your extra income.
If you are a freelancer or sell items online, you will fill out the Self-Employment section.
If you rent a room or a house, you will fill out the Property Income section.
Type in your total gross turnover (all the money you brought in) and deduct your allowable business expenses to calculate your net profit.
4. How HMRC Calculates Your Tax Bill
Once you submit your figures, HMRC's automated system combines your P60 salary with your side-hustle profits to calculate your overall tax position.
Because your full-time job likely uses up your tax-free Personal Allowance (£12,570), your side-hustle profits will usually be taxed immediately at your current tax band.
If you are a standard-rate taxpayer, your extra profits will be taxed at 20%.
If your combined job salary and side hustle profit pushes you over £50,270, the profits above that line will be taxed at the higher rate of 40%.
The system will subtract the tax your employer already paid via PAYE and show you the remaining balance you owe for your extra earnings.
5. Pay Your Tax Bill Without Missing the Deadline
Just like any other taxpayer, you must submit your online form and pay the tax you owe by 31 January following the end of the tax year.
If you file your tax return early (before 30 December) and your tax bill is under £3,000, you can often choose to have HMRC collect the tax automatically by adjusting your full-time job tax code for the next year. This spreads your tax payments across 12 months rather than paying a lump sum in January.