Register on GOV.UK by 5 October after the end of the tax year you start. Self-employed plumbers earning over £1,000 in a tax year must register. Choose "working as a subcontractor" to add CIS for contractor installs (GOV.UK, October 2026).
Self Assessment is how HMRC taxes your plumbing profits. You register online. HMRC then sends you a Unique Taxpayer Reference (UTR), and you send a tax return for each year. This page follows GOV.UK as checked on 18 October 2026.
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When a plumber has to register
| Your situation | What you register |
|---|---|
| Self-employed, earning more than £1,000 in a tax year before expenses | Self Assessment, as a sole trader |
| Installing heating or water systems for a builder or other contractor | Self Assessment and CIS together, by choosing "working as a subcontractor" |
| Only repairs, servicing and boiler swaps for homeowners | Self Assessment only; CIS does not apply to homeowners |
| Employed on PAYE, plus your own jobs over £1,000 a year | Self Assessment for the side work |
| Registered before but no return was needed for 2024-25 | You may need to reactivate your account instead |
You must tell HMRC by 5 October if you need a return for the previous tax year and have not sent one before, or registered before but did not need one for 2024-25. For 2025-26 that was 5 October 2026. If you start in 2026-27, it is 5 October 2027.
Where CIS fits for plumbers
Work for homeowners who pay you directly is outside CIS. CIS 340, HMRC's guide to the scheme, says private householders are not contractors, so a homeowner deducts nothing.
Work for a contractor is different. CIS 340 counts installing heating, drainage, sanitation and water supply systems in a building as construction work. In our reading, that puts first fix and second fix on a new build, for a developer or a builder, inside CIS. CIS 340 also says that when a builder is doing a homeowner's job and pays you for included work on it, the builder must operate CIS on its payments to you.
Some plumbing is outside CIS even for a contractor. CIS 340 leaves out repair, servicing and maintenance of heating and water systems, and replacing parts such as taps or a radiator. HMRC's CIS manual (CISR14090) says replacing a boiler on its own is not installing a system, while replacing the whole system is. If one contract mixes excluded and included work, every payment under it is inside CIS. Our guide does CIS apply to plumbers and heating engineers has more examples.
If any contractor will pay you for installs, choose "working as a subcontractor" when you register. GOV.UK says that registers you for Self Assessment and CIS at the same time. If you are registered and the contractor can verify you, it deducts 20%, not counting VAT and materials you paid for. If not, it deducts 30%. See how to register for CIS as a plumber.
Steps to register
- Check you are self-employed for this work, not an employee of a firm.
- Find your National Insurance number, which GOV.UK says you need.
- Go to GOV.UK's "Check how to register for Self Assessment" page and say you are self-employed.
- Choose "working as a subcontractor" if a contractor will pay you for installation work.
- Answer the questions about you and your business, including when you started trading, then submit.
- Wait for your UTR. GOV.UK says it will be in your personal tax account within 72 hours if you register online, or usually 15 working days by post.
- Give each contractor the exact name you registered with, your UTR and your National Insurance number. GOV.UK says contractors need all three to verify a sole trader, and the details must match your registration exactly.
- Keep records of every job and every CIS statement from the start.
- Send your first return online by 31 January after the tax year ends, and pay what you owe by then.
What this means for you
Registering also opens the door to Making Tax Digital for Income Tax (MTD). GOV.UK says you can only sign up for MTD once you are registered for Self Assessment and have sent a return in the last 2 years.
MTD looks at your self-employment and property income before expenses, from your last return. In our reading, CIS work counts at its gross pay, before deductions. If that was over £30,000 on your 2025-26 return, MTD starts for you on 6 April 2027. If it was over £50,000 on your 2024-25 return, it should already have started on 6 April 2026.
Use the free MTD check to see your date, and the trades section for more guides for plumbers.
A worked example
Callum leaves a heating firm in April 2026 to work for himself. Most weeks he services boilers and fixes leaks for homeowners. Every few weeks a local builder pays him for bathroom installs on extensions. He registers online in May 2026 and chooses "working as a subcontractor" because of the builder's work.
That is well before his 5 October 2027 deadline. His UTR is in his personal tax account within 72 hours, and the builder verifies him and deducts 20%. Callum's first return, for 2026-27, is due online by 31 January 2028. It includes his homeowner income and the builder's gross payments, with the CIS taken off counted against his bill.
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Questions people ask
I mostly fix leaks and service boilers for homeowners. Do I still need to register?
Yes, if you earn more than £1,000 from self-employment in a tax year before expenses. CIS 340 says private householders are not contractors, so CIS will not apply to that work, but it is still self-employed income for your tax return.
Do I need CIS as a plumber?
Only for construction work paid by a contractor, such as installing heating or water systems on a builder's job. CIS 340 leaves out repair, servicing and maintenance of heating and water systems, unless they are in the same contract as CIS work. If you will do any contractor installs, choose 'working as a subcontractor' when you register.
I am a Gas Safe engineer. Is that my tax registration?
No. GOV.UK says you must be on the Gas Safe Register to do gas work legally. That is a safety register, not a tax one. Self Assessment is your tax registration with HMRC, and you need it separately if you are self-employed.
I started in 2025-26 and missed 5 October 2026. What happens?
Register as soon as you can. GOV.UK says you could get a penalty if you tell HMRC after 5 October 2026. HMRC gives you 3 months from the date of its letter or email to file the 2025-26 return, but the tax is still due by 31 January 2027.
When will my UTR arrive?
GOV.UK says that if you register online it will be in your personal tax account within 72 hours. By post it usually takes 15 working days, and longer if you live abroad. It is a 10-digit number. You need it to register for CIS online, and contractors use it to verify you.
Sources
- Check how to register for Self Assessment (GOV.UK)
- Become a sole trader: Register as a sole trader (GOV.UK)
- Self Assessment tax returns: Who must send a tax return (GOV.UK)
- What you must do as a CIS subcontractor: How to register (GOV.UK)
- Construction Industry Scheme: a guide for contractors and subcontractors, CIS 340, sections 1.2 and 2.3 and Appendices A and C (GOV.UK)
- CISR14090: The Scheme: construction operations: building service systems (HMRC manual)
- What you must do as a CIS subcontractor: Get paid (GOV.UK)
- Self Assessment tax returns: Deadlines (GOV.UK)
- Find your Unique Taxpayer Reference (UTR) (GOV.UK)
- Sign up for Making Tax Digital for Income Tax (GOV.UK)
- Find out if and when you need to use Making Tax Digital for Income Tax (GOV.UK)
- Register to carry out gas work (GOV.UK)
- What you must do as a CIS contractor: Verify subcontractors (GOV.UK)
This is general information to help you understand the rules. It isn't tailored to your circumstances, so check your own position before acting on it.
Righto Tax is a trading name of NJG Consultancy Ltd, registered in England and Wales, company number 16095900. Registered office: Jubilee House, East Beach, Lytham St Annes, FY8 5FT.
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