Why Register at All? You Pay Less Tax Upfront

CIS registration costs nothing and takes around 10 minutes. The reason to do it is purely financial: registered subcontractors have 20% deducted from their labour invoices by contractors. Unregistered subcontractors have 30% deducted.

On every £1,000 of labour you invoice, registration saves you £100 in cash now. You will reconcile the difference at year-end through Self Assessment — but having that money in your account during the year materially improves your cashflow.

What Work Counts as Construction

CIS applies to most construction operations carried out on a permanent or temporary building or structure. The full list HMRC uses is broad, but for trades it covers the obvious work:

Site preparation, including groundworks and demolition
Building, structural alterations, repairs, decorating, and dismantling
Installation of systems for heating, lighting, power, water, drainage, and ventilation
Cleaning the inside of buildings after construction work
Civil engineering, including roads, railways, harbours, and pipelines

Work That Is Specifically Excluded

Not everything done on a construction site is CIS work. The following are explicitly excluded and the standard PAYE/self-employment rules apply instead:

Architecture and surveying
Manufacture or delivery of materials only (no on-site installation)
Carpet fitting (despite often being done alongside other construction trades)
Sign-making and installation
Scaffolding hire without labour

What You Need to Register

HMRC asks for the same information regardless of business structure. Have these ready before you start the form:

Your Unique Taxpayer Reference (UTR) — issued when you registered for Self Assessment
Your National Insurance number
Your business name and trading address
Your business legal structure (sole trader, partnership, or limited company)
Your business bank account details (sort code and account number) so contractors can verify you

After You Register

Once HMRC has processed your registration (usually within 24–48 hours), contractors who are about to pay you will run a verification check against HMRC's system. The check returns your CIS rate — 20% for registered, 30% for unregistered, or 0% if you hold gross payment status.

Each contractor only needs to verify you once. You keep your verification number for any future work with the same contractor — though if you go more than two tax years without a payment from them, they have to verify you again.

You will still need to file a Self Assessment tax return each year. The CIS deductions show as advance payments of your income tax and Class 4 NI; they are netted off the total tax you owe (or refunded if you have overpaid).

Common Registration Mistakes

Registration itself is hard to get wrong, but there are a couple of points where new subcontractors lose money:

Not separating materials from labour on invoices — CIS deductions only apply to the labour portion. If you do not break it down, contractors deduct on the full amount and you wait until year-end to reclaim.
Mixing trading names — register the same legal entity you use to invoice. A different trading name on an invoice can fail HMRC verification and force the contractor to apply 30%.
Forgetting to update bank details — contractors verify against HMRC records, including the bank account. A mismatched account triggers a higher deduction rate or a refusal to pay.

Getting the Workflow Right from Day One

The cleanest setup, before your first CIS invoice goes out, is to use an invoicing tool that already understands the CIS workflow — separates materials from labour, applies the correct deduction rate per customer, and produces compliant payment & deduction statements automatically. Manual spreadsheets work until you have a handful of customers, then break.