If you work in construction — whether you're laying foundations in Birmingham or fitting kitchens in Edinburgh — there's a good chance the Construction Industry Scheme (CIS) applies to you. Yet despite affecting hundreds of thousands of businesses across the UK, CIS remains one of the most misunderstood areas of tax. Contractors miss registration deadlines, subcontractors get taxed at the wrong rate, and HMRC penalties quietly stack up. This guide cuts through the jargon and tells you exactly what you need to know, whether you're a contractor, a subcontractor, or an accountant managing both.
What Is CIS and Who Does It Apply To?
The Construction Industry Scheme is an HMRC tax deduction scheme that has been in place, in various forms, since the 1970s. Its core purpose is straightforward: to reduce tax evasion in the construction sector by requiring contractors to deduct money from subcontractors' payments and pass it directly to HMRC on their behalf.
The scheme applies to businesses and individuals working in the construction industry in the UK. Crucially, HMRC defines "construction" broadly. It's not just bricks and mortar. CIS covers:
- Site preparation, excavation, and groundwork
- Building, alteration, repair, and demolition
- Installation of heating, lighting, water, and ventilation systems
- Painting and decorating (where it forms part of construction work)
- Landscaping and civil engineering
It does not apply to architecture, surveying, carpet-fitting as a standalone trade, or purely professional services. If you're a sole trader plumber subcontracting for a larger building firm, CIS almost certainly applies to you. If you're a freelance architect drawing up plans, it probably doesn't.
There are two roles within CIS: contractors and subcontractors. A contractor is a business that pays subcontractors for construction work. A subcontractor is a business or individual that carries out that work. You can be both simultaneously — for example, a main contractor who subcontracts specialist trades.
Registering for CIS: What Contractors and Subcontractors Must Do
Registration requirements differ depending on your role.
Contractors must register for CIS with HMRC before taking on their first subcontractor. You do this through your Government Gateway account. Once registered, you're obligated to verify your subcontractors with HMRC before making any payment — more on that below.
Subcontractors don't have to register, but failing to do so is costly. An unregistered subcontractor has 30% deducted from their payments. A registered subcontractor pays just 20%. And if you've applied for and been granted gross payment status, you receive your full payment with no deduction at all — a significant cash flow advantage for established businesses.
To qualify for gross payment status, HMRC requires that your business passes three tests: a business test, a turnover test (at least £30,000 net for a sole trader or £30,000 per director for a company), and a compliance test, meaning your tax affairs must be up to date. HMRC reviews gross payment status periodically, so it's not something you earn once and forget.
How CIS Deductions Work in Practice
Here's where many contractors come unstuck. Before paying a subcontractor for the first time, you must verify them through HMRC. You can do this online via the Government Gateway or by calling the CIS helpline on 0300 200 3210. HMRC will tell you whether the subcontractor is registered, unregistered, or holds gross payment status — and this determines the deduction rate: 20%, 30%, or 0%.
CIS deductions apply only to the labour element of an invoice. Materials — such as timber, pipe fittings, or plasterboard — are excluded, provided they're itemised separately. So if a subcontractor invoices £2,000 for labour and £500 for materials, you deduct 20% of £2,000 (£400), not the full £2,500.
Each month, contractors must submit a CIS monthly return to HMRC, listing every subcontractor paid and the deductions made. The deadline is the 19th of the following month (or 22nd for electronic payment). Missing this deadline triggers automatic penalties — £100 for the first month, rising steeply thereafter. You must also give each subcontractor a payment and deduction statement (sometimes called a CIS voucher) showing how much was deducted.
Subcontractors, in turn, use those deduction statements to offset what they owe when they submit their Self Assessment tax return or company tax return. Essentially, the deductions function as advance payments of tax — so you shouldn't be paying the same liability twice.
Common CIS Mistakes and How to Avoid Them
The same errors crop up time and again, and HMRC is not sympathetic when they do.
- Failing to verify subcontractors before payment. If you pay a subcontractor without verifying them first, HMRC may hold you liable for the correct deduction even if you didn't make one.
- Deducting from materials. Applying CIS deductions to the materials portion of an invoice is a common accounting error that causes headaches when subcontractors reconcile their tax position.
- Missing the monthly return deadline. Even if you've made no payments to subcontractors that month, you must still file a nil return or inform HMRC you're inactive.
- Incorrectly classifying workers as subcontractors. HMRC's employment status rules are strict. Paying someone as a CIS subcontractor when they should legally be an employee exposes you to significant back-tax liability. Use HMRC's Check Employment Status for Tax (CEST) tool if you're unsure.
- Losing track of deduction statements. Subcontractors who cannot produce their CIS payment statements at Self Assessment time risk overpaying tax. Keep records meticulously.
Good record-keeping is the single most effective defence against CIS compliance issues. If you're managing multiple subcontractors, tracking verifications, deduction rates, and monthly returns manually in a spreadsheet is a recipe for errors. Tools like BizHub365 can help here — its accounting module allows you to track invoices, log deductions against the correct labour and materials split, and maintain the audit trail HMRC expects, all within the same platform you use for your wider business finances.
CIS Refunds: Getting Back What You're Owed
One underappreciated aspect of CIS is that many subcontractors are entitled to a refund. Because deductions are taken at source throughout the year, your total deductions often exceed your actual tax liability — particularly if your business has significant allowable expenses.
Sole trader subcontractors reclaim overpaid CIS deductions through their annual Self Assessment tax return. Limited company subcontractors reclaim via their company tax return or by offsetting against PAYE/NI liabilities if they run a payroll. HMRC does not automatically alert you to a potential refund; you need to claim it actively.
The key is speed and accuracy. Ensuring your payment and deduction statements match what HMRC holds on record is essential — discrepancies cause delays. If you're an accountant managing subcontractor clients, building a systematic process for collecting CIS statements ahead of each filing season will save you considerable time.
Conclusion: Stay Organised, Stay Compliant
CIS is not especially complicated once you understand the underlying logic: it's a system designed to collect tax early and reduce evasion in a cash-heavy industry. The scheme becomes problematic only when businesses ignore registration requirements, miss monthly filing deadlines, or fail to keep accurate records.
Whether you're a sole trader subcontractor trying to understand why 20% keeps disappearing from your invoices, or a contractor managing a roster of trades, the principles are the same: verify before you pay, deduct from labour only, file on time every month, and keep every piece of paperwork. Get those fundamentals right, and CIS becomes a manageable part of running your construction business — not a source of unwelcome surprises from HMRC.