If you work in construction in London, as a self-employed subcontractor or a contractor paying subbies, the Construction Industry Scheme (CIS) applies to you. And getting it wrong is more expensive than most people realise.

Here’s what a CIS accountant actually does, and why it matters.

What is CIS?

The Construction Industry Scheme is HMRC’s system for collecting tax from construction workers at source. If you’re a subcontractor, your contractor deducts either 20% or 30% from your payments before you ever see them. That money goes straight to HMRC as a tax advance.

The problem? Many subcontractors end up overpaying throughout the year and don’t realise they can claim a refund. Others underpay because their deductions weren’t set up correctly. Both situations are costly.

What a CIS Accountant Does For You

For subcontractors:

• Files your annual Self-Assessment tax return, including all CIS deductions suffered
• Calculates whether you’re owed a rebate, and claims it back for you
• Makes sure your expenses (materials, tools, travel, PPE) are correctly deducted
• Keeps you compliant so you stay on the 20% deduction rate, not 30%

For contractors:

• Verifies your subcontractors with HMRC before paying them
• Prepares and submits your monthly CIS returns on time
• Handles any HMRC queries or investigations
• Advises on when to employ vs subcontract

Common CIS Mistakes I See

Subcontractors not registered with HMRC. If you’re not registered, contractors must deduct 30% from your pay rather than 20%. That’s a 10% difference that adds up fast on a £5,000 monthly invoice.

Contractors not verifying subcontractors. This creates liability for the contractor if the wrong deduction rate is applied.

Missing the self-assessment deadline. CIS subcontractors must file a Self-Assessment return annually. Missing the 31 January deadline triggers immediate penalties.

Not tracking expenses. Tools, materials, mileage, protective clothing — these all reduce your taxable income. But you need records.

How Much Can You Get Back?

It depends on your situation, but I regularly secure CIS rebates for subcontractors in the £1,000–£5,000 range. In some cases, much more, especially where deductions have been applied at the higher 30% rate or expenses haven’t been claimed properly.

I’m Talha Alvi, a CIS-specialist accountant based in Stanmore. I work with contractors and subcontractors across London, from Harrow and Wembley to Edgware and beyond. Fixed fees, quick turnaround, no phone tag. Get in touch at info@yourtaxhelp.co.uk.