Welcome back to Week 9 of our 'Small Business Finance and CIS Compliance Essentials' series! If you’ve been following along, you’ll know we’ve been diving deep into the world of construction finance. I’m Claire Murphy, and here at SBS Essex Ltd, we spend a lot of time helping contractors and subcontractors navigate the often-confusing waters of the Construction Industry Scheme (CIS).
Let’s be honest: CIS is a bit of a headache. It’s one of those things that feels like a mountain of paperwork standing between you and the actual work you want to be doing on-site. But, as an AAT Licensed Practitioner, I’ve seen firsthand how quickly a small oversight can turn into a massive fine from HMRC.
The good news? Most CIS mistakes are completely avoidable. Today, I’m breaking down the seven most common blunders we see in the construction sector and, more importantly, how you can fix them before the taxman comes knocking.
1. Playing the Guessing Game: "Is This Work CIS?"
One of the biggest mistakes people make is assuming they know which jobs fall under CIS and which don’t. It sounds simple, if it’s construction, it’s CIS, right? Well, not exactly. HMRC has very specific (and sometimes weird) rules about what counts.
For example, did you know that simple maintenance: like cleaning the windows of a new build: might fall under CIS if it’s part of the finishing process? Or that landscaping only falls under CIS if it’s part of a larger construction project? If you’re just doing a bit of gardening for a residential client, it’s out. But if you’re turfing a lawn as part of a new housing development, you’re in.
How to fix it:
Stop guessing. If you’re unsure, check the HMRC CIS guide or, better yet, give us a shout. It’s always better to over-verify than to find out three years later that you should have been deducting tax. At SBS Essex Ltd, we specialise in identifying the scope of works so you don’t have to play detective.
2. Skipping the Verification Step
I see this all the time: a contractor hires a "mate" or a regular subcontractor and just assumes they know their tax status. They might have been "Gross Payment Status" three years ago, so the contractor doesn't bother checking.
Big mistake. A subcontractor’s status can change in the blink of an eye. If they miss their own tax return or fall behind on payments, HMRC might revoke their Gross Payment Status. If you pay them without checking and only deduct 0% (instead of the 20% or 30% they should have been on), you are the one liable for the difference.
How to fix it:
Verify every single subcontractor before you make the first payment: and re-verify them regularly. You can do this through the HMRC online portal, or if you’re using our outsourced bookkeeping UK services, we handle the entire verification process for you. We make sure you’re applying the right rate every single time.
3. The "Mixed Contract" Trap
This is a sneaky one. Let’s say you have a contract that includes a bit of construction work and a whole lot of something else (like delivering materials or consultancy). Many businesses think they can split the invoice: applying CIS to the construction bit and paying the rest normally.
Nope. HMRC doesn't like that. If a contract includes any construction operations, the entire payment for that contract (minus materials) falls under CIS. This is what we call a "mixed contract."
How to fix it:
If the job involves any construction elements, treat the whole thing as a CIS contract. It’s the safest way to stay compliant. Trying to "carve out" non-CIS elements is a surefire way to trigger an audit.
4. Getting Creative with Material Costs
As a contractor, you’re allowed to deduct the cost of materials before you calculate the CIS tax. But some people get a bit… "creative" here. You can only deduct the actual cost of materials that the subcontractor paid for. You can't deduct things like tool hire, fuel, or scaffolding costs unless they are specifically part of the materials used in the permanent structure.
If your subcontractor puts "Materials: £1,000" on an invoice, you should really be asking for the receipts. If HMRC finds out those "materials" were actually just a new set of drills, they’ll want the tax you didn’t deduct.
How to fix it:
Insist on seeing a breakdown of material costs on every invoice. Keep those records safe. Proper bookkeeping services for small business will always ensure that deductions are backed up by evidence. It’s not about being difficult; it’s about protecting your business from future penalties.
5. Forgetting the "Deemed Contractor" Rule
Think you’re safe because you aren’t a "construction company"? Think again. If your business spends a lot of money on construction: say you’re a pub chain or a large office-based business: and you spend more than £3 million on construction operations over a rolling 12-month period, you become a "Deemed Contractor."
This means you have the same responsibilities as a building firm: you have to register for CIS, verify your tradespeople, and file monthly returns.
How to fix it:
Keep a close eye on your annual spend if you’re doing major renovations or site work. If you’re approaching that £3 million mark, you need to register for CIS immediately. Our team at sbsessex.co.uk can help you track this threshold so you’re never caught off guard.
6. Missing the Monthly Return Deadline
The CIS deadline is the 19th of every month. It doesn't matter if it’s a bank holiday, your birthday, or if the dog ate your laptop: HMRC wants that return. If you’re even one day late, there’s an automatic £100 penalty. If you’re two months late, it’s another £200. It adds up fast.
The biggest mistake here? Not filing a "Nil Return." If you didn’t pay any subcontractors this month, you still have to tell HMRC that you paid zero. If you just don’t file, they’ll assume you’ve forgotten and send you a fine anyway.
How to fix it:
Set a reminder for the 19th of every month or, better yet, automate the process. Most modern accounting software handles this, but if you want total peace of mind, outsourcing your bookkeeping is the way to go. We haven't missed a deadline yet!
7. Poor Record-Keeping (The Silent Killer)
In an HMRC inspection, the burden of proof is on you. If you can’t show the verification reference, the invoice breakdown, or the proof of payment, HMRC will assume the worst. "I lost the receipts in the van" isn't a valid legal defence, unfortunately.
Construction sites are messy, and paperwork often gets lost in the cab of a lorry or covered in plaster dust. But failing to keep organised records is the easiest way to lose a CIS dispute.
How to fix it:
Go digital! Use an app to snap photos of receipts and invoices the moment you get them. Cloud-based bookkeeping allows us to see your figures in real-time, meaning we can spot errors before they become permanent records.
Why Compliance Matters More Than Ever
In 2026, HMRC is more joined-up than ever. Their systems talk to each other, and they are cracking down on the construction sector specifically. Being "mostly right" isn't enough anymore. You need to be 100% right.
At SBS Essex Ltd, we don’t just "do the books." As an AAT Licensed Practitioner, I take pride in helping our clients build solid financial foundations. Whether you’re a sole trader just starting out or a large contractor with a fleet of vans, we offer tailored bookkeeping services for small business that take the weight off your shoulders.
Choosing outsourced bookkeeping UK means you get to focus on the build, while we focus on the bills. We’ll handle the verifications, calculate the deductions, and ensure your monthly returns are filed correctly and on time.
Let’s Get Your CIS Sorted
If any of these seven mistakes rang a bell, don’t panic! It’s never too late to get your house in order. We’re here to help you organise your finances and stay on the right side of HMRC.
Check out our website at sbsessex.co.uk to see how we can support your construction business. Let’s make sure your next HMRC notification is a confirmation of your return, not a penalty notice.
Stay tuned for Week 10, where we’ll be talking about how to manage cash flow when your payments are tied up in the CIS system!
Cheers,
Claire Murphy
Founder, SBS Essex Ltd
