SiteKiln gives general information, not legal, tax or financial advice. Talk to a qualified professional before making big decisions.
# Part-Time Self-Employment: Starting a Trade Alongside a Job
Loads of sparks, plumbers and chippies start out doing weekend and evening work while keeping their day job. It's the smart way to build a client base and test the water before going full-time. This guide covers the tax, insurance and legal bits you need to get right.
Rule of thumb: if you're earning anything from private work outside your PAYE job, HMRC needs to know about it. The trading allowance gives you the first £1,000 tax-free, but after that you need to register as self-employed.
The Tax Side
You'll have two income streams. Your employer handles PAYE and takes tax and NI from your wages automatically. Your self-employed income sits on top of that.
Both go on the same Self-Assessment tax return. Your PAYE income uses up your Personal Allowance first (£12,570 for 2025-26), so most of your self-employed profit gets taxed at 20% from the first pound. If your combined income pushes above £50,270, the extra gets taxed at 40%.
The trading allowance gives you the first £1,000 of self-employed income completely tax-free. If you only do the odd job here and there and earn under a grand a year, you don't even need to register. Once you go above that, register with HMRC within three months.
Tip for new starters: the trading allowance is useful in your first few months when you're just picking up small jobs. But once you start spending money on materials, tools and van costs, you're usually better off registering properly and claiming actual expenses instead. You can't do both.
National Insurance: Watch for Overpayment
This catches people out. You pay Class 1 NI through your employment (deducted from your wages). You also pay Class 2 and Class 4 NI on your self-employed profits.
Class 2 is now "deemed paid" if your profits are above £12,570 (2025-26 rates). Class 4 is 6% on profits between £12,570 and £50,270, then 2% above that.
Because you're paying NI from two sources, you can sometimes overpay. If your employed earnings already hit the Upper Earnings Limit (£50,270), you may be able to defer Class 4 contributions. Check with HMRC or your accountant.
Check Your Employment Contract
Before you take on a single private job, read your employment contract. Look for:
- Restrictive covenants. Some contracts say you can't work for competitors or solicit clients within a certain area.
- Moonlighting clauses. Some employers require written permission for any outside work.
- Non-compete clauses. Rare in construction but they exist, especially if you're employed by a specialist firm.
If your contract says nothing about outside work, you're probably fine. If it does, talk to your employer. Most are reasonable about it, especially if your private work isn't competing directly with them.
Insurance: Your Employer's Cover Does Not Protect You
This is the big one. Your employer's public liability and professional indemnity insurance covers you while you're working for them, on their jobs, under their supervision. The moment you pick up a drill on a Saturday for your own customer, none of that applies.
You need your own:
- Public liability insurance - covers damage to a customer's property or injury to a third party. Minimum £1m, but £2m is standard.
- Professional indemnity insurance - covers you if your work causes a problem later (a leak, an electrical fault, structural issues).
Some insurers offer part-time or limited-hours policies that cost less than full-time cover. Shop around. Don't skip this.
CSCS Card
Your employer's CSCS card arrangements don't cover your private work. Check what card you hold and whether it's valid for the type of work you're doing privately. If you're a qualified spark doing domestic rewires on weekends, you need the right card for that scope of work, not just the one your employer arranged for commercial sites.
Tip for new starters: if you're planning to go full-time self-employed eventually, sort your own CSCS card now while you've still got a steady income. It removes one barrier when you make the jump.
When to Make the Jump
There's no magic number, but here's a sensible test. When your weekend work is consistently bringing in more than your day job minus expenses, and you've got three months of bills saved up, you're in a strong position to go full-time. Not before.
Three months of living costs in the bank covers the gap while you build up. Construction work is seasonal and lumpy. January to March can be dead. Having a buffer stops you taking bad jobs from bad clients just to keep the lights on.
Setting Up Properly
Even while you're part-time, treat it like a real business from day one:
- Register with HMRC as self-employed
- Open a separate bank account for your trade income
- Keep every receipt
- Get your own insurance
- Use proper quotes and invoices
- Register for CIS if you're doing subcontract work
The habits you build now carry forward when you go full-time.
Sources
- HMRC, "Self-Assessment guidance for individuals with multiple income sources," 2025-26
- HMRC, "National Insurance: Class 2 and Class 4 rates," 2025-26
- gov.uk, "Trading allowance," 2025-26
- gov.uk, "Employment contracts and restrictive covenants"
- HSE, "Insurance requirements for self-employed contractors"
Know someone who needs this?
Was this guide useful?
Didn't find what you were looking for?
Spotted something wrong or out of date? Email us at hello@kilnguides.co.uk.
In crisis? Samaritans 116 123 ·