How CIS Subcontractors Accidentally Overpay Thousands

Every year thousands of UK construction workers overpay tax.

Not by a few pounds. Sometimes by hundreds or even thousands of pounds.

And the frustrating part? Most subcontractors never realise it’s happening.

Under the Construction Industry Scheme (CIS), contractors deduct tax from subcontractors before paying them. The idea is simple: HMRC collects tax in advance so people working in construction don’t fall behind.

In practice, the system often does the opposite. Subcontractors end up paying too much tax upfront, then struggle to reclaim what they’re owed.

Missed expenses, poor record-keeping and confusion about CIS deductions mean a lot of people are quietly handing more money to HMRC than they should.

How CIS Tax Actually Works

According to GOV.UK CIS guidance, if a subcontractor is not registered with HMRC for the Construction Industry Scheme, contractors must deduct 30% from their payments. Registered subcontractors have 20% deducted, while those with gross payment status have 0% deducted.

Under CIS, contractors deduct tax from subcontractor payments before passing the rest on.

The standard rates are:

• 20% deduction for registered subcontractors
• 30% deduction if you’re not registered
• 0% deduction if you have gross payment status

These deductions count as advance payments toward your tax bill.

But here’s the catch.

The deduction is based on your labour payment, not your actual profit. That means the system ignores things like:

• Tools
• Travel
• Materials
• Training
• Insurance
• Equipment

Those costs only get accounted for when you submit your tax return. Until then, HMRC may be holding far more tax than you actually owe.

Why Subcontractors Often Overpay

There are three common reasons CIS subcontractors end up paying too much tax.

1. The deduction ignores expenses

Construction workers often have significant work costs. Fuel, tools, van repairs, protective equipment and training all add up quickly.

But CIS deductions are calculated before expenses are considered.

So if you receive £30,000 in labour payments but spend £10,000 on legitimate work costs, the tax deducted during the year may be based on the full £30,000.

That means you’ve effectively paid tax on money you never kept.

2. Expenses are forgotten

Even when subcontractors know they can claim expenses, many forget large chunks of them. Construction work often involves:

• Travel between multiple sites
• Buying small tools and materials
• Replacing equipment
• Training courses
• Professional fees

When receipts aren’t tracked properly during the year, many of these costs never get claimed. And that directly increases the tax bill.

3. CIS deductions aren’t recorded properly

Another surprisingly common issue is missing CIS deductions. Every contractor should provide a monthly CIS deduction statement, showing:

• The amount paid
• The CIS tax deducted

But subcontractors often:

• Lose statements
• Don’t collect them
• Don’t record them properly

If the deduction isn’t recorded in the tax return, HMRC assumes it was never paid. Which means the subcontractor ends up paying that tax again.

The Real Numbers: How Much Is Being Missed?

The UK construction industry has over 900,000 CIS subcontractors, according to HMRC estimates.

Research from accounting firms and industry groups consistently shows that construction workers frequently miss legitimate expense claims.

Common estimates suggest subcontractors may underclaim £1,000 to £3,000 in expenses each year simply because records aren’t kept properly.

And when CIS deductions are already being taken during the year, that overpayment can sit with HMRC until the tax return is submitted.

The 10 CIS Expenses Subcontractors Forget Most Often

When we look at construction tax returns, the same expenses appear again and again. And the same ones get missed.

None of these are unusual. They’re part of everyday construction work. But if they’re not recorded during the year, they’re incredibly easy to forget when tax season arrives.

The Hidden Problem With “Doing It Later”

Many subcontractors leave their bookkeeping until the end of the tax year.

It’s understandable. When you’ve been working long days on site, sitting down with receipts in the evening is the last thing anyone wants.

But this approach creates two problems.

First, receipts disappear. A fuel receipt from ten months ago might technically be claimable, but if it’s buried somewhere in the van or thrown away, it never makes it into the tax return.

Second, details get forgotten. Multiply that by dozens of small expenses and the missed tax savings add up quickly.

Why CIS Refunds Can Take So Long

Because CIS deductions are taken throughout the year, many subcontractors end up due a refund when their tax return is submitted.

But that refund only appears once HMRC processes the return and verifies the deductions. If information is missing, the process slows down.

That’s why many subcontractors feel like they’re constantly waiting for money that technically belonged to them in the first place.

The Simple Way to Stop Overpaying

The good news is that the solution isn’t complicated. It mostly comes down to record-keeping during the year, not scrambling at the end of it.

Subcontractors who track their income and expenses consistently usually find that:

• They claim more legitimate expenses
• They avoid missing CIS deductions
• Their tax bill becomes more predictable
• Refunds are processed faster

Instead of guessing at the end of the year, everything is already recorded.

Where 123Tax Fits In

Construction workers didn’t get into the trade because they enjoy tax admin. But the way CIS works means keeping track of your numbers properly can make a real financial difference.

123Tax is designed to make that process easier. Instead of dealing with piles of receipts and spreadsheets, subcontractors can:

• Track income as it comes in
• Capture expenses during the year
• Record CIS deductions properly
• Stay ready for tax returns and future MTD updates

The result is simple. Less paperwork, fewer surprises, and less tax paid unnecessarily.

Summary

The Construction Industry Scheme is designed to collect tax in advance from subcontractors, but the system often results in overpayments during the year.

Because CIS deductions are calculated before expenses are considered, many subcontractors end up paying tax on income that doesn’t reflect their real profit.

Missed expenses, poor record-keeping and unrecorded CIS deductions can all increase the final tax bill.

With hundreds of thousands of subcontractors working under CIS across the UK, even small mistakes can lead to significant overpayments each year.

Tracking expenses and deductions consistently during the year makes it far easier to claim the correct tax relief and avoid paying more tax than necessary.