Case Study: No Concrete in the Concrete Floor — Ruislip

Of the five case studies in Your Seven Step Home Extension Plan, this is the one people don't believe at first. Ashton didn't believe it either. It started with a phone call from Bill, a business coach he'd known for a few years:

"I have some close friends who are really lovely people. They've recently refurbished their house in North London, and they've just been informed that the builder hasn't put concrete in the concrete floor."

No concrete. In the concrete floor. And the builder wanted another £10,000 from the clients to put it right.

The background

Mrs Cullen had inherited the family home when her mother passed away — a good-sized semi-detached house in Ruislip. She and her husband engaged a design and build project management company to extend and refurbish the house, and all had gone well.

Then the flooring subcontractor rang them. The message: check the concrete floor — with a hint that it didn't contain any concrete.

By the time Ashton spoke to Mrs Cullen, the family had spent all their money, and the builder was asking for another £10,000 to resolve the problem — presented as though it were somehow the client's fault. She was in tears on the phone, apologising for a situation she hadn't created.

How do you build a floor with no concrete in it?

To understand the defect, picture a concrete ground floor as a layer cake. For the build-up specified on this extension, the listed layers were:

• 150 mm broken-brick hardcore
• 50 mm sand blinding
• 150 mm concrete
• Damp proof membrane
• 75 mm insulation, with the underfloor heating within the build-up
• 75 mm floor screed
• ~20 mm floor tiles and grout

The listed depths total about 520 mm, plus the membrane. An excavation only 450 mm deep was therefore around 70 mm too shallow for the full specified build-up. That established that the floor could not have been assembled as specified, but it did not prove from the measurements alone which layer was missing.

The shallow excavation was evidence of a defect, not proof by itself that the concrete had been omitted or why. That is why the later strategy called for a trial hole to establish the floor's actual construction. It subsequently transpired that there had been a payment disagreement between the main contractor and the subcontractor — and the subcontractor had deliberately sabotaged the project. The homeowners were collateral damage in someone else's dispute.

The rescue

The first professional move wasn't a site visit — it was paperwork. Send me everything: drawings, approvals, emails, letters, contracts, quotes, names and contact numbers. If in doubt, email it. A full file was set up, printed and reviewed before ever meeting the clients — then a Tuesday-evening drive up to Ruislip.

The good news: the Cullens had a written contract — a Federation of Master Builders (FMB) plain-English contract with the design and build contractor. It was found to be limited, but it only just provided sufficient grounds for a claim: the main contractor's duty of care to properly supervise his subcontractor, and his negligence in supervising the excavation for the floor.

The strategy had several strands worth copying:

Straight talk about cost and time. The Cullens were told from the start: this could take six months or more to resolve, it could go legal, with architectural costs of £5,000–£10,000 and legal costs double that. No false comfort.

Keep the client involved. Mr Cullen was asked to run credit checks on the relevant companies to check their financial position and trading records — partly for the claim, partly so the family didn't sit feeling helpless. (His best friend happened to be a barrister, who reviewed the findings and strategy.) One real risk in cases like this is the contractor folding the company and escaping the claim entirely, so the pressure had to be firm but calibrated.

Prove the defect, don't assert it. The letter to the builders stated the belief that the kitchen floor had not been constructed properly and requested a small trial hole or pilot hole to determine the floor's thickness — did it or did it not have a 150 mm concrete layer?

Price the real remedy. The builder's offer to fix his own defect for £10,000 plus VAT — payable by the client — was, in the book's words, ballsy. The true remedial scope was calculated at £35,000 plus VAT: removal and storage of the £15,000 kitchen, a temporary kitchen or temporary accommodation during the works, breaking out and re-excavating the existing floor to the required depth, constructing the new concrete floor correctly, new underfloor heating pipework, and redecoration of the kitchen ceilings, walls and joinery that would inevitably be damaged during the works.

Set a deadline with consequences. If no clear remedial strategy arrived within 14 days, legal proceedings would start and the builder's insurers would be notified.

The letter had the desired effect. The builders checked the floor, confirmed it didn't match the specification and drawings, and put a solution on the table — initially short on detail, so it was expanded until it properly protected the Cullens. Liability accepted, remedial works at no cost to the client.

The builders had one condition: that Ashton be removed from the proceedings. With the defect identified, liability accepted and a solution agreed, there was no objection — though the Cullens were warned they were only halfway, because the remedial works themselves could present a challenge. Three months later, the works still weren't complete and the builders were still messing them about — but the family were confident they were over the worst.

Where did they go wrong?

The book's post-mortem lists four decisions to learn from:

• Design and build with a single company. The package looked attractive, but with only one company involved there's no independent voice when something goes wrong. The traditional model — independent designer, separate builder — provides more autonomy. (It's why our own surveyor-led approach keeps professional oversight and inspection at the centre of every project rather than leaving the builder to mark his own homework.)
• The contract choice. The FMB contract was better than no contract, but in Ashton's opinion the JCT Minor Works contract would have provided much better protection and a proper vehicle for dispute resolution. The contract you sign on a good day is the tool you'll reach for on your worst day — choose it accordingly.
• Recommendation without due diligence. The Cullens were impressed by a glossy portfolio and a charismatic owner. If you're spending £180,000 of hard-earned savings or equity, follow up recommendations, check previous projects and run due diligence on the company — the same checks we set out in how to find a good builder.
• Subcontractors. Most builders rely on subcontractors, because they can't afford to employ every trade — which makes payment of those subcontractors critically important to your project. The recommendation: either have individual contracts with subcontractors or make sure they are named in the contract. On this project, a payment dispute the clients knew nothing about nearly cost them a £35,000 floor.

What this story shows

A hidden defect under a finished kitchen floor is close to the worst case in residential building — the work looked complete, the money was spent, and the failure was invisible until someone with inside knowledge spoke up. What saved the Cullens was not luck: it was a written contract (however imperfect), a full paper trail, an independent professional to price the true remedy, and a strategy that made fixing the floor cheaper for the builder than fighting about it. It's also a reminder of why independent inspection during the build — checking compliance against drawings and specification while the layers of that cake are still visible — is worth so much more than any argument after the event.

If you're planning an extension, the time to set up those protections is before the first dig, not after the first phone call from a subcontractor. The full story — and the chapters on paperwork, problems and managing your builder that flow from it — is in Your Seven Step Home Extension Plan.

Worried about a project — or setting one up properly?

Whether you're partway through a build that doesn't feel right, or setting up an extension and want the contract, specification and inspections done properly from day one, book a free consultation and we'll give you an honest, independent view.