How To Register As Self Employed UK While Keeping Your Full Time Job

If you have a full time job and you are earning money on the side through freelance work, a small business, or a side hustle, you need to know exactly when HMRC expects you to register as self employed. The answer is not "as soon as you earn a penny". There is a specific trigger, and getting it wrong means either paying penalties or missing out on tax relief you are entitled to.

This guide covers the practical steps for registering as self employed in the UK while keeping your full time job. It includes the trading allowance rules, how National Insurance works when you have two income sources, and what happens to your tax code. The figures are for the 2025/26 tax year unless stated otherwise.

When Must You Register As Self Employed UK?

You must register with HMRC as self employed if your self employed income exceeds the trading allowance of £1,000 in a tax year. The trading allowance is a tax-free amount you can earn from self employment or casual income without needing to tell HMRC or pay tax on it.

If your gross self employed income is £1,000 or less in a tax year, you do not need to register. You also do not need to file a self assessment return for that income. HMRC treats this as de minimis income that falls outside the tax system entirely.

The moment your gross income from self employment passes £1,000, you must register by 5 October following the end of the tax year in which you exceeded the threshold. For example, if you started a side business in June 2025 and your income hits £1,200 by March 2026, you must register by 5 October 2026.

There is a common misconception that you can register at the same time as filing your self assessment return. You cannot. The registration deadline is 5 October. The filing deadline is 31 January. Missing the registration deadline means a penalty, even if you file the return on time.

How To Register As Self Employed UK Online

Registration is done through HMRC's online service. You need a Government Gateway user ID and password. If you already have one for PAYE (because of your full time job), you can use the same login.

Here is the step-by-step process:

• Go to GOV.UK and search for "register for self assessment".
• Select the option to register as self employed.
• Enter your personal details, National Insurance number, and contact information.
• Provide a description of your self employed business activity.
• Give a start date for your self employment. This is the date you started trading, not the date you registered.
• Confirm whether your turnover is expected to be above or below the VAT threshold of £90,000.

Once registered, HMRC will issue you a Unique Taxpayer Reference (UTR) if you do not already have one. You will also receive a notice to file a self assessment return each year. You must file a return even if your self employed income is below the personal allowance, because you have exceeded the trading allowance and HMRC needs to see the figures.

If you already have a UTR from previous self employment or from being a company director, you do not need a new one. You simply add the new self employment to your existing self assessment record. You can do this through your HMRC online account under "add a new source of income".

What About the Trading Allowance When You Have a Full Time Job?

The trading allowance is £1,000 per tax year. It applies to your gross self employed income before any expenses. If your gross income is £1,000 or less, you owe no tax and do not need to register.

If your gross income exceeds £1,000, you have a choice. You can either deduct your actual expenses and pay tax on the profit, or you can use the £1,000 trading allowance instead of deducting expenses. You cannot use both for the same trade.

Here is how the choice works with a real example. Say you earn £2,500 from freelance graphic design work in 2025/26. Your actual expenses are £400 for software subscriptions and £150 for a domain and hosting. Your profit using actual expenses is £1,950. Using the trading allowance, your taxable profit is £1,500. In this case, the trading allowance saves you tax.

But if your expenses were £1,800, your actual profit would be £700. Using the trading allowance would give you £1,500 taxable profit, which is worse. You would choose to deduct actual expenses instead.

The trading allowance is particularly useful for side hustles with low overheads. A freelance consultant in Manchester doing occasional project work with no direct costs will almost always benefit from using the allowance.

National Insurance When You Have a Full Time Job and Self Employment

This is where most people get confused. Your full time job pays Class 1 National Insurance through PAYE. Your self employment triggers Class 2 and Class 4 National Insurance. But the rules changed in 2024/25.

For 2025/26, Class 2 National Insurance is effectively abolished for most self employed people. If your self employed profits are above £6,725, you are treated as having paid Class 2 NIC, which counts towards your National Insurance record for state pension and benefits. You do not pay anything. If your profits are below £6,725, you can choose to pay voluntary Class 2 NIC to protect your record.

Class 4 National Insurance is charged at 6% on self employed profits between £12,570 and £50,270, and at 2% on profits above £50,270. However, your full time job income uses up your primary threshold. This means your Class 4 NIC starts from the first pound of self employed profit, not from £12,570.

Let us work through an example. You earn £35,000 from your full time job. You earn £8,000 profit from self employment. Your total income is £43,000. Your Class 4 NIC on the self employment is 6% of £8,000, which is £480. The £12,570 threshold has already been used by your employment income, so there is no gap to fill.

If your full time job income is below £12,570, the self employed profits fill the gap up to the threshold, and Class 4 NIC only applies to profits above £12,570. This is rare for someone with a full time job, but it happens for part time workers with a side business.

Income Tax on Side Hustle Income

Your self employed profit is added to your employment income and taxed at your marginal rate. There is no separate tax calculation. The self assessment return combines everything.

Using the same example: £35,000 employment income plus £8,000 self employed profit equals £43,000 total income. The personal allowance of £12,570 covers part of the employment income. The basic rate band of £37,700 (up to £50,270) covers the rest. Your self employed profit is taxed at 20% basic rate, so £1,600 in income tax on the side income.

If your total income pushes you into the higher rate band above £50,270, the self employed profit above that threshold is taxed at 40%. A side hustle earning £20,000 profit on top of a £45,000 salary gives total income of £65,000. The first £5,270 of side income falls in the basic rate band. The remaining £14,730 is taxed at 40%. That is £1,054 at 20% and £5,892 at 40%, totalling £6,946 in income tax on the side income.

You can reduce this by claiming allowable expenses. Travel costs, equipment, software, professional fees, and a portion of home costs (if you work from home) are all deductible. Keep receipts and records. HMRC can ask to see them.

Do You Need To Tell Your Employer?

Your employment contract may have a clause restricting outside business activities. Check it before you start. Many contracts include a "moonlighting" clause that requires you to get written permission before taking on any other paid work. Some sectors, particularly financial services and civil service, have strict rules about secondary employment.

If your contract is silent on the issue, you are generally free to take on self employed work as long as it does not create a conflict of interest, use your employer's resources without permission, or compete with your employer's business. A freelance graphic designer working for a design agency should not take clients that compete with the agency. A software developer building a side app while employed by a tech company should check their intellectual property assignment clause.

HMRC does not tell your employer that you have registered as self employed. The tax system treats your employment and self employment as separate. Your employer sees only your PAYE code, which may change if HMRC adjusts it to collect tax on your side income. That change could prompt questions, but it is not a direct notification.

How Your Tax Code Changes

When you file your self assessment return, HMRC may adjust your PAYE tax code for the following year to collect any underpaid tax from your side income. This is common if your self employed profit is relatively small and HMRC wants to collect the tax through your employment rather than asking for a lump sum payment.

If HMRC adjusts your tax code, you will receive a PAYE coding notice (form P2). It will show a reduction in your personal allowance or an adjustment to collect the estimated tax. You can challenge this if you prefer to pay the tax through your self assessment instead, but most people find the tax code adjustment convenient.

For larger self employed profits, HMRC will not adjust the tax code. They will expect you to make payments on account through the self assessment system. Payments on account are due on 31 January and 31 July each year, each being half of your previous year's tax bill (excluding tax already collected through PAYE).

Record Keeping for a Side Hustle

You must keep records of your self employed income and expenses for at least 5 years after the 31 January filing deadline. HMRC can request these records during a compliance check. Digital records are fine. Spreadsheets, accounting software like FreeAgent or Xero, or even a well-organised folder of receipts and invoices all count.

For a side hustle, you do not need to use Making Tax Digital for Income Tax until April 2026 if your qualifying income is over £50,000, April 2027 if over £30,000, and April 2028 if over £20,000. If your side income is below £20,000, you are not required to use MTD-compatible software for at least several years. You can still use it voluntarily if you prefer.

Most side hustlers with a full time job find FreeAgent or GoSimpleTax sufficient for record keeping. These tools also calculate your tax liability and generate the figures for your self assessment return.

What If Your Side Hustle Grows?

If your self employed income grows significantly, you may need to consider VAT registration. The VAT threshold is £90,000 of taxable turnover in a rolling 12-month period. If your side business passes that threshold, you must register for VAT and charge VAT on your invoices. This applies regardless of your full time job.

You may also consider incorporating as a limited company if your profits are consistently above £40,000 to £50,000. The tax advantages of a limited company versus sole trader status depend on your specific circumstances, including how much profit you need to draw and whether you want to retain earnings in the business. Our incorporation and structure guide covers the decision in more detail.

If you do incorporate, you will need to close your self employment registration with HMRC and start filing company accounts and corporation tax returns instead. The process is straightforward but requires careful timing to avoid overlapping tax obligations.

Common Mistakes To Avoid

Three mistakes come up repeatedly with clients who start a side business while employed.

First, registering too late. The 5 October deadline catches people out every year. If you started trading in September 2025 and your income passed £1,000 by March 2026, you must register by 5 October 2026. If you wait until January 2027 when you file your return, you have missed the deadline and face a penalty.

Second, not keeping separate bank accounts. HMRC can and does ask to see bank statements during compliance checks. Mixing personal and business transactions makes it harder to prove your expenses and income. A separate business bank account is not legally required for a sole trader, but it makes your life significantly easier. Monzo, Starling, and Tide all offer free or low-cost business accounts suitable for side hustles.

Third, ignoring the trading allowance when it works in your favour. Many people deduct actual expenses without checking whether the £1,000 allowance would give a better result. Run both calculations each year and use the one that reduces your tax bill.

When To Speak to an Accountant

Most side hustles with straightforward income and expenses do not need an accountant. The self assessment process is manageable with good records and HMRC's online guidance.

But if your side income crosses £10,000, if you have complex expenses like a home office calculation, if you are unsure about IR35 status for freelance contracts, or if you are considering incorporation, speak to a qualified accountant. Our services page covers the range of support we offer, and we are happy to have an initial conversation about your situation.

If your side business involves construction work, you may need to register under the Construction Industry Scheme (CIS). That adds another layer of compliance. And if you are a contractor working through an umbrella company or your own limited company, the rules are different from straightforward self employment. Our limited company tax section covers contractor-specific guidance.

Registering as self employed while keeping your full time job is a straightforward process. Know the £1,000 trading allowance trigger, register by 5 October, keep good records, and file your return by 31 January. Do that and you stay on the right side of HMRC while building your side income.