CIS Scheme for Plumbers: Complete Guide for Contractors and Subcontractors

The Construction Industry Scheme (CIS) is an HMRC tax scheme that regulates how contractors pay subcontractors for construction work. Under CIS, contractors deduct money from a subcontractor's payment and pass it directly to HMRC as an advance payment towards the subcontractor's tax and National Insurance bill.

CIS applies to plumbing work in the following circumstances:

• You are a subcontractor doing plumbing work for a main contractor on a construction project
• You work on new builds, major refurbishments, commercial fit-outs, or any project where another party (the contractor) manages the overall job
• The work involves construction operations as defined by HMRC — which includes plumbing, heating, drainage, and related mechanical work on a permanent structure

CIS does not generally apply to:

• Direct work for a homeowner (you are paid gross by the homeowner — normal self-assessment tax applies)
• Maintenance and repair work that is incidental to a larger project
• Businesses not registered for CIS acting purely as domestic service providers

If you subcontract plumbing work to another plumber (making you a contractor in CIS terms), CIS obligations apply to you as the paying party, regardless of the size of your business. A sole trader plumber who regularly hires a mate as a subcontractor is a CIS contractor and must register and operate CIS correctly.

CIS has two distinct registration roles and you may be both simultaneously:

Registering as a CIS subcontractor

If you do plumbing work for contractors, you should register as a subcontractor with HMRC. Registration is free and done through the HMRC website or by calling the CIS helpline. You will need your UTR (Unique Taxpayer Reference) from your Self Assessment registration.

If you are registered as a subcontractor, contractors will deduct 20% from your labour payments as a standard CIS deduction and pay it to HMRC on your behalf. This is not a final tax — it is an advance payment credited against your annual tax bill.

If you are not registered as a subcontractor and a contractor tries to verify you, HMRC will tell them to deduct 30% — a higher rate for unregistered subcontractors. Registering is therefore strongly in your financial interest.

Registering as a CIS contractor

If you pay subcontractors for construction work, you must register as a contractor with HMRC, regardless of your business size. Registration is through the HMRC CIS helpline or online. Once registered, you have monthly obligations (see below).

How to register: Visit gov.uk/what-is-the-construction-industry-scheme or call HMRC CIS helpline on 0300 200 3210. Have your UTR, National Insurance number, and business details ready.

Understanding what CIS deductions apply to is essential to avoid deducting the wrong amount — a common source of disputes between plumbing contractors and subcontractors.

CIS deductions apply to labour only — not to materials. If a plumbing subcontractor's invoice is £2,000 for labour and £800 for materials (pipe, fittings, valves), the CIS deduction is calculated only on the £2,000 labour element:

• Standard deduction (registered subcontractor): £2,000 × 20% = £400 deducted, subcontractor receives £1,600 labour + £800 materials = £2,400
• Higher deduction (unregistered subcontractor): £2,000 × 30% = £600 deducted

Materials must be genuinely the subcontractor's cost — materials bought by the contractor and supplied to the subcontractor are not excluded from the deduction calculation. The materials element must represent costs the subcontractor directly incurred.

VAT is excluded from CIS calculations — if the subcontractor is VAT-registered, CIS deductions are calculated on the net (ex-VAT) labour amount. VAT is paid separately and in full.

Verifying subcontractors before payment: Contractors must verify a subcontractor with HMRC before making the first payment. This is done online through the HMRC CIS online service or by calling the helpline. Verification tells you whether to deduct at 20%, 30%, or 0% (gross payment status). You must re-verify each subcontractor at least once per tax year.

If you are a subcontractor, CIS deductions are an advance on your tax bill — you get the money back when you file your Self Assessment return. But having 20% deducted from every invoice creates a cash flow problem, particularly when large jobs are involved.

Gross payment status (GPS) allows subcontractors to receive the full payment without any CIS deduction. The contractor verifies your status and HMRC tells them to pay you gross. You are then responsible for paying your own tax through Self Assessment in the normal way.

To qualify for gross payment status, you must meet HMRC's tests:

• Business test: You must be carrying on a business in the UK that includes construction work
• Turnover test: Your net annual turnover from construction work (excluding materials and VAT) must reach certain thresholds — for sole traders, HMRC sets the threshold; for partnerships and companies the figure is higher. Check current thresholds at gov.uk
• Compliance test: Your tax affairs must be in order — no late returns, no unpaid tax, no significant compliance failures in the past 12 months

Applying for gross payment status: Apply online through your HMRC business tax account. HMRC reviews your compliance history and turnover records. If approved, it takes effect from the date HMRC grants it and lasts until your next annual review.

Maintaining gross payment status: HMRC reviews GPS holders annually and will remove the status if your compliance record slips. Filing Self Assessment returns late, paying tax late, or failing to operate CIS correctly as a contractor are all grounds for removal. Losing GPS means returning to 20% deductions immediately.

If you are registered as a CIS contractor, you must submit a monthly return to HMRC — even in months where you make no subcontractor payments. Failing to file returns is a significant compliance failure and can affect your own gross payment status if you hold it.

What a monthly CIS return includes:

• Each subcontractor you paid that month
• The total amount paid (labour only, ex-VAT)
• The amount deducted (at 20% or 30%)
• Whether the subcontractor is registered or unregistered with CIS

Deadlines: The monthly return must be submitted by the 19th of the following month. So payments made in June must be returned by 19 July. CIS deducted must be paid to HMRC by the same date. Late returns attract automatic penalties — £100 for missing the deadline, increasing for longer delays.

How to file: CIS monthly returns are filed through HMRC's online CIS service at gov.uk or through compatible payroll/accounting software. Most bookkeeping software (Xero, QuickBooks, FreeAgent) handles CIS returns if set up correctly.

Nil returns: If you make no subcontractor payments in a month, you must still file a nil return by the 19th. Failing to file nil returns generates the same penalties as failing to file a return with actual payments.

Subcontractor statements: After deducting CIS, you must give each subcontractor a written statement showing the gross amount, deduction amount, and net amount paid. This is their evidence for claiming the deduction back through Self Assessment. Provide it promptly — subcontractors need it for their own records.

CIS is an area where small mistakes create large penalties. These are the most common errors specific to plumbing businesses:

1. Not registering as a contractor when hiring labour — many small plumbing businesses use a regular subcontractor without realising this creates CIS obligations. If you regularly pay the same individual for plumbing labour on construction projects, CIS almost certainly applies. The employment status question (employee vs subcontractor) is separate — you can have a genuine self-employed subcontractor and still owe CIS obligations.

2. Deducting CIS from materials — the deduction applies to labour only. Deducting 20% from the full invoice including materials overpays HMRC and underpays the subcontractor. Keep invoices clearly itemised between labour and materials.

3. Not verifying subcontractors before paying — if you fail to verify and pay a 20% deduction to someone who should be 30% (unregistered), HMRC may require you to make up the shortfall. Always verify through the HMRC CIS service before the first payment.

4. Filing returns late or not at all — penalties are automatic and start at £100 per month. A year of missed returns generates significant penalty debt even if the actual CIS deductions owed are modest.

5. Assuming CIS doesn't apply to small jobs — there is no de minimis threshold. A £500 plumbing job on a construction site requires the same CIS compliance as a £50,000 contract.

6. Failing to provide deduction statements — not giving subcontractors a written statement of their deduction means they cannot accurately claim it back. This is a legal obligation, not optional.

If CIS has been deducted from your payments throughout the year, those deductions are credited against your Self Assessment tax liability. In practice, this often means plumbing subcontractors receive a tax refund at the end of the year.

How the process works:

1. Collect your CIS deduction statements from every contractor who deducted from you throughout the year
2. Total up the gross labour payments and the deductions made
3. Include these on your Self Assessment return in the CIS section
4. HMRC credits the deductions against your income tax and Class 4 National Insurance liability
5. If deductions exceed your liability (common in a slow year or if you have significant expenses to offset), HMRC issues a refund

If a contractor failed to give you a deduction statement: Request it formally in writing. HMRC can in some cases use PAYE records to verify deductions made, but having your own statements is much simpler. Keep copies of all deduction statements permanently — they are your evidence if HMRC queries your Self Assessment return.

If you are also VAT-registered: Remember that CIS deductions are calculated on net (ex-VAT) labour. When you reconcile your year, the figures you use for Self Assessment are net of VAT. VAT is accounted for separately through your VAT return.

If you find CIS administration complex, a bookkeeper or accountant familiar with the construction industry is worth the fee — particularly if you operate as both contractor and subcontractor simultaneously, which is common for mid-sized plumbing businesses.