How Does CIS Work?

At Towerstone Accountants we provide specialist CIS accountancy services for contractors and construction businesses across the UK. We created this webpage for subcontractors and contractors who want clear guidance on the Construction Industry Scheme, including registration, deductions, refunds, and ongoing reporting obligations. Our aim is to help you stay compliant with HMRC, avoid costly errors, and understand how CIS affects your tax position.

When I speak to contractors and subcontractors for the first time one of the most common reactions I hear is confusion around CIS. People know money is being deducted, they see figures on statements that do not quite line up with what they invoiced, and they are never fully sure whether CIS is a tax they have paid, a tax they still owe, or something sitting in the background waiting to catch them out later.

In my experience CIS is not complicated, but it is detailed, procedural, and very unforgiving if you misunderstand how it works. Once you understand the logic behind it, it becomes far easier to manage, plan for, and use properly as part of your wider tax position.

In this article I am going to explain how CIS works in practice, not just in theory. I will walk through what CIS is, who it applies to, how deductions are calculated, how it affects your tax return, and the mistakes I see most often. My aim is that by the end you will understand exactly where you stand, what you should be doing, and why CIS exists in the first place.

What CIS actually is and why it exists

The Construction Industry Scheme, usually shortened to CIS, is a tax deduction scheme run by HMRC. It applies to construction work carried out in the UK and is designed to collect tax at source, meaning tax is deducted before the subcontractor receives their money.

HMRC introduced CIS because historically the construction industry had high levels of unpaid tax. Work was often short term, subcontractors moved frequently between jobs, and income was not always declared properly. CIS shifts part of the responsibility for tax collection onto contractors, which makes it harder for income to go unreported.

From a practical point of view CIS is not an extra tax. It is a way of paying tax in advance. If it is handled correctly the deductions you suffer through the year are set against your final tax bill.

Who CIS applies to

CIS applies to two main groups of people, contractors and subcontractors.

A contractor is a business that pays subcontractors to carry out construction work. This includes traditional builders but it can also include property developers, landlords carrying out large works, and businesses outside construction if construction work is a significant part of their activity.

A subcontractor is a business or individual who carries out construction work for a contractor. This can be a sole trader, a partnership, or a limited company.

Construction work under CIS is defined quite broadly. It includes building, alterations, repairs, demolition, civil engineering, and installation work such as plumbing, electrics, heating, and air conditioning. It does not generally include professional services like architecture or surveying, or the manufacture of materials off site.

Registering for CIS

If you are a contractor you must register with HMRC for CIS before you pay any subcontractors. This is not optional. Registration allows you to verify subcontractors, deduct tax where required, submit monthly returns, and make payments to HMRC.

If you are a subcontractor you should also register for CIS. Registration determines the rate at which tax is deducted from your payments and whether tax is deducted at all.

This is one of the first areas where I see problems arise. Subcontractors assume CIS registration is automatic or that it is handled by the contractor. It is not. You must actively register yourself.

CIS deduction rates explained

There are three possible deduction positions under CIS.

The standard deduction rate is 20 percent. This applies to subcontractors who are registered with CIS.

The higher deduction rate is 30 percent. This applies if the subcontractor is not registered with CIS.

The third option is gross payment status. If approved, the subcontractor is paid in full with no CIS deductions taken at all.

It is important to understand that these deductions apply only to the labour element of the invoice. Materials, VAT, and certain other costs are excluded from the deduction calculation.

How subcontractor verification works

Before paying a subcontractor the contractor must verify them with HMRC. This is done using the subcontractor’s details, usually their name, UTR, and business type.

Verification tells the contractor which deduction rate to apply. Until verification is completed the contractor must deduct tax at 30 percent.

In practice this means that if your details are wrong or incomplete you can end up losing a significant portion of your payment unnecessarily, which can cause cash flow issues very quickly.

How CIS deductions are calculated

This is an area where confusion is very common.

CIS deductions are calculated on the labour element of the invoice only. The basic steps are:.

• Take the total invoice value

• Remove VAT if charged

• Remove the cost of materials

• Apply the CIS deduction rate to the remaining labour amount

For example if you invoice £2,000 plus VAT and £500 of that relates to materials, the CIS deduction is calculated on £1,500. If you are on the 20 percent rate the deduction would be £300.

This is why it is essential to clearly separate labour and materials on your invoices. If you do not, contractors are entitled to treat the whole amount as labour.

How CIS works for limited companies

One of the most misunderstood areas of CIS is how it applies to limited companies.

A limited company subcontractor can be registered for CIS and can suffer deductions at 20 percent or 30 percent, or apply for gross payment status. CIS still applies even though the company pays Corporation Tax rather than Income Tax.

The key difference is how CIS deductions are later offset. For limited companies CIS deductions are set against PAYE liabilities and Corporation Tax, not personal tax.

In many cases I see limited companies build up large CIS credits because they have low PAYE bills and do not claim refunds promptly.

Gross payment status explained

Gross payment status allows subcontractors to be paid in full with no CIS deductions. This can significantly improve cash flow.

However it is not automatic and it comes with strict conditions. To qualify you must:.

• Run a genuine construction business

• Meet turnover thresholds

• Have a good compliance record with HMRC

HMRC regularly reviews gross status and can remove it if returns are late or tax is unpaid. Losing gross status can be financially painful and often comes as a surprise to those who have not kept on top of compliance.

From experience I always advise clients with gross status to treat compliance as non negotiable.

Monthly CIS returns for contractors

Contractors must submit a CIS return every month. This shows all payments made to subcontractors and the deductions taken.

The return must be submitted by the 19th of the following month. Even if no payments were made a nil return is still required.

Late returns trigger automatic penalties. These start small but quickly escalate. I regularly deal with cases where penalties have reached thousands of pounds simply because returns were overlooked.

Paying CIS deductions to HMRC

CIS deductions must be paid to HMRC along with PAYE and National Insurance by the 22nd of the following month if paying electronically.

These deductions are not the contractor’s money. They are held on trust for HMRC. Using them to fund cash flow is extremely risky and often leads to serious problems.

How CIS affects your Self Assessment tax return

For sole traders and partnerships CIS deductions are treated as tax already paid.

When I prepare Self Assessment returns for CIS subcontractors I include the gross income and allowable expenses as normal, then offset the CIS deductions against the tax and National Insurance due.

In some cases the deductions exceed the final tax bill, resulting in a refund. In other cases there is still tax to pay, particularly if profits are high or if there are other income sources.

CIS refunds and why they take time

CIS refunds are common, particularly for subcontractors with low profits or significant expenses.

However refunds are not instant. HMRC will only issue a refund once the tax return has been processed and sometimes additional checks are carried out.

One of the most important things I do for clients is ensure CIS deduction statements are accurate and complete. Missing or incorrect statements can delay refunds for months.

CIS and VAT

CIS and VAT are separate systems but they interact in practical ways.

VAT is never subject to CIS deductions. This means that even if 20 percent is deducted from labour, VAT is still paid in full.

For subcontractors this can create cash flow pressure, particularly for those on standard VAT accounting. In some cases the Flat Rate Scheme or cash accounting can help, but this must be reviewed carefully.

Common CIS mistakes I see

After years of working with CIS clients there are recurring issues I see again and again.

These include not registering for CIS, invoicing incorrectly, misunderstanding what deductions mean, failing to keep statements, and assuming CIS replaces the need for a tax return.

CIS does not remove your obligation to submit returns. It does not guarantee you have paid the right amount of tax. It is simply part of the process.

How CIS works when you have other income

Many subcontractors have other income such as employment, property income, or dividends.

CIS deductions only relate to construction income. When calculating your final tax position all income sources are combined. This means CIS deductions may only cover part of your overall liability.

This is why planning is important. CIS can create a false sense of security if you assume everything is already covered.

What happens if CIS is done incorrectly

Errors under CIS can be costly.

Contractors can be fined for failing to verify subcontractors or submit returns. Subcontractors can lose gross status or face delays in refunds.

In serious cases HMRC can open compliance checks or investigations. These are time consuming and stressful but often avoidable with proper systems in place.

Keeping records under CIS

Good record keeping makes CIS manageable.

You should keep invoices, CIS statements, bank records, and correspondence with contractors. For contractors this also includes verification records and monthly returns.

Digital record keeping makes this far easier and aligns with HMRC’s broader direction of travel.

How I help clients manage CIS properly

When I work with CIS clients my focus is always on clarity and control.

I make sure registrations are correct, deductions are understood, records are complete, and tax returns are prepared accurately. I also help clients assess whether gross status is appropriate and how CIS fits into their wider tax planning.

CIS does not need to be a source of anxiety. Once it is set up correctly it becomes routine.

Final thoughts

CIS can feel intimidating at first but at its core it is a straightforward system designed to collect tax earlier rather than later. The problems arise when people do not understand how it fits into their overall tax position or when compliance slips.

In my experience the subcontractors who struggle most with CIS are not those earning the least, but those who assume it will take care of itself. With the right understanding, proper registration, and accurate reporting, CIS becomes predictable and manageable.

You may also find our guidance on what is cis and what is cis registered helpful when dealing with related CIS questions. For a broader overview of CIS rules, compliance, and support, you can visit our CIS guidance hub.