How do I register for Self Assessment as a builder?

Register online on GOV.UK by 5 October after the tax year you started. As a new self-employed builder paid by contractors, choose "working as a subcontractor" and you are registered for Self Assessment and CIS together (GOV.UK, October 2026).

Self Assessment is how HMRC collects Income Tax and National Insurance from self-employed builders. You register online. HMRC then sends you a Unique Taxpayer Reference (UTR) and you send a tax return each year. This page follows GOV.UK as checked on 18 October 2026.

When a builder has to register

The tax year runs from 6 April to 5 April. If you first became self-employed in 2025-26, the deadline to tell HMRC was 5 October 2026. If you start during 2026-27, it is 5 October 2027.

Building work for homeowners counts too. CIS 340, HMRC's guide to the scheme, says private householders are not contractors, so a homeowner deducts nothing from you. That money is still self-employed income, and it goes on your return with your CIS work.

The CIS shortcut for builders

GOV.UK's CIS guide says the online CIS registration needs a UTR. If you have no UTR yet, you register as a new business for Self Assessment and choose "working as a subcontractor" when prompted. You are then registered for Self Assessment and CIS at the same time, with net payment status. That means a contractor who can verify you deducts 20%, but not from VAT or the materials you paid for.

If you skip CIS or the contractor cannot match you, HMRC tells them to deduct 30%. The deductions are not lost. They count towards your tax bill, and anything taken over what you owe is paid back after your tax return. But 30% leaves you short on every payment until the return is done.

If you also pay other trades, such as a self-employed plasterer or labourer, you need a second registration as a CIS contractor. That goes through HMRC's new-employer process, not Self Assessment. Our guide to registering for CIS as a builder covers both sides.

Steps to register

1. Check you are self-employed rather than an employee of the firm you work for.
2. Find your National Insurance number. You need it to register.
3. Go to GOV.UK's "Check how to register for Self Assessment" page and choose "You're self-employed".
4. When asked, choose "working as a subcontractor" if any contractor will pay you for construction work.
5. Answer the questions about you and your business, including when you started trading, then submit.
6. 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.
7. Give every 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.
8. Keep records of your income, CIS statements and costs from day one.
9. Send your first return online by 31 January after the tax year ends, and pay what you owe by the same date.

You will also need a sign-in for HMRC online services to file your return and, later, to use Making Tax Digital.

What this means for you

Registering is also the gate to Making Tax Digital for Income Tax (MTD). To sign up for MTD you must be registered for Self Assessment and have sent a return in the last 2 years.

Your CIS pay before deductions goes on your return as income, so in our reading it counts towards the MTD income test, together with your homeowner work and any rental income. If that came to more than £30,000 before expenses 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.

So a builder who started in 2025-26 needs three things in order: the registration, the 2025-26 return by 31 January 2027, and then MTD from April 2027 if the income is over the line. Use the free MTD check to see where you stand, and the trades section for more on CIS and starting out.

A worked example

Dean leaves a payroll job in June 2025 to work for himself. A main contractor pays him for groundworks, and he does two patios for homeowners. He registers online in July 2025, choosing "working as a subcontractor". Once his UTR is in his personal tax account, the contractor verifies him, and deductions drop from 30% to 20%.

Dean's 2025-26 income before costs is £34,000: £28,000 gross from the contractor and £6,000 from homeowners. He files his return by 31 January 2027, and HMRC takes his CIS deductions off his bill and repays any excess. Because £34,000 is over £30,000, MTD starts for him on 6 April 2027. His first year is only about 10 months, and GOV.UK says HMRC scales a short first year up to a full year, so he is over the line either way.