Choosing the wrong builder is one of the most expensive mistakes a homeowner can make. In Hertfordshire, where a house extension costs £55,000–£120,000 and a loft conversion costs £38,000–£68,000, the difference between a well-vetted contractor and a poorly chosen one can be the difference between a project that completes on time and on budget, and one that leaves you with an unfinished structure and a legal dispute.
And here's the uncomfortable truth: the building industry in England is largely unregulated. Anyone can call themselves a builder, take a deposit, and start work. There's no mandatory licensing, no minimum qualification requirement, and no central register of competent contractors. That's not a criticism of the industry — it's a fact that shapes how you need to approach the selection process.
This guide gives you the practical vetting framework TCM uses when we're asked how to evaluate a contractor. It covers the Companies House check, the three insurance policies that are non-negotiable, the seven red flags that should make you walk away, and — as the Step 7 anchor — TCM's own contract structure formatted as a checklist you can use to evaluate any builder you're considering.
Industry Context
According to the Federation of Master Builders' State of Trade Survey 2024, 40% of homeowners who experienced problems with a building project had not checked the contractor's insurance before appointing them. That single omission accounts for a disproportionate share of the disputes that end up in court or with Citizens Advice. Which?'s 2025 consumer survey found that 1 in 4 homeowners who used an unvetted contractor reported problems — compared to 1 in 12 for those who used a verified trade platform.
What Makes a Builder Trustworthy? The Three Non-Negotiables
A trustworthy builder has three things: a verifiable legal identity, adequate insurance, and a track record of completed work you can inspect or verify. Everything else — accreditations, memberships, website quality, van livery — is secondary. The vetting process in this guide is designed to confirm all three before you sign anything.
Think about it this way. A builder's website can be built in a weekend. A Checkatrade profile can be created in an hour. But a Companies House record showing ten years of filed accounts, a current insurance certificate with a £5 million limit, and a list of completed projects with contactable clients — those take years to build and can't be faked.
| Trade | Certification Requirement | Risk Without It |
|---|---|---|
| Gas installation | Gas Safe Register — legally required | Illegal and dangerous without |
| Electrical work | Part P competent person scheme | Legal liability without certification |
| General building | No mandatory licence | Vetting is entirely buyer's responsibility |
| Structural work | Building Regulations approval | Enforcement notice if bypassed |
| Asbestos removal | HSE licensed contractor | Criminal offence without licence |
| Party wall work | No licence, but legal process | Injunction risk without agreement |
The Vetting Process: Step by Step
Don't start with a quote. Start with the checks. Here's the order that matters.
Before you ask for a quote, check that the company exists as a legal entity. Go to companieshouse.gov.uk and search for the company name. A legitimate limited company will have a registration number, a registered address, filed accounts, and a list of directors.
Check when the company was incorporated. A company registered last month with no filed accounts is a very different proposition to one that's been trading for ten years with clean accounts. Also check the Insolvency Register at gov.uk to confirm there are no active insolvency proceedings against the company or its directors.
For sole traders, ask for a UTR (Unique Taxpayer Reference) number and verify they're registered for Self Assessment with HMRC. It's a basic indicator of legitimacy, but it matters.
TCM's Details
TCM Building & Maintenance Ltd — Company Number: 09044688. Incorporated 19 May 2014. Accounts filed consistently since incorporation. Search us on Companies House before you call.
Seven Red Flags: Walk Away If You See These
Experience teaches you to recognise the warning signs early. Here are the seven most reliable indicators that a contractor is not the right choice for your project. Honestly, if you see more than one of these, don't proceed — the risk isn't worth it.
1. Requests a large upfront deposit
A legitimate contractor with a healthy cash flow doesn't need 30–50% of the contract value before work starts. A reasonable deposit is 10–15%, with staged payments tied to programme milestones. Large upfront payment requests are the single most common precursor to contractor fraud, according to Citizens Advice's 2025 consumer fraud data.
2. Can't provide insurance certificates
If a contractor says their insurer will send the certificate 'next week' or offers a verbal assurance instead, walk away. Insurance certificates are issued immediately upon policy inception and should be available on request within 24 hours. No certificate means no insurance.
3. Pressures you to decide quickly
'I've got a gap in my schedule next week but I need to know by tomorrow' is a sales technique, not a genuine scheduling constraint. A contractor who pressures you to sign before you've had time to check references and compare quotes is not operating in your interest.
4. Suggests avoiding VAT by paying cash
This is tax evasion. If a contractor suggests it, they're asking you to participate in an illegal arrangement. It also means the contractor has no incentive to provide a paper trail for the work, which makes disputes much harder to resolve.
5. Can't show you completed projects
Every experienced contractor has a portfolio of completed work they're proud of. If a contractor can't show you at least three completed projects — ideally with contact details for the clients — that's a significant gap. It means either they're new, or their previous clients wouldn't recommend them.
6. Refuses to use a written contract
Any project over £1,000 should have a written contract. For a house extension or loft conversion, a JCT Minor Works Building Contract is the industry standard. A contractor who refuses to use a written contract, or who offers only a one-page quote-and-acceptance document, is not protecting your interests.
7. Vague about subcontractors and their insurance
Most builders use subcontractors for specialist trades — groundworks, steelwork, roofing, electrical, plumbing. That's normal. What matters is whether the main contractor takes responsibility for the subcontractors' work and whether those subcontractors are also insured. Ask directly: 'Who will be carrying out the structural work, and are they covered by your insurance?'
TCM Contract Checklist
12 Items TCM Includes in Every Residential Contract
This is the exact structure TCM uses for every residential project over £10,000. Use it as a checklist when evaluating any builder you're considering. If a contractor's contract is missing more than two of these items, ask why — or ask them to add them before you sign.
1. Scope of works
A written description of exactly what is and isn't included. Vague scope is how extras get added.
2. Contract sum
The fixed price, or a clear mechanism for calculating the final price if it's a cost-plus contract.
3. Programme
Start date, end date, and key milestones. Not just 'approximately 12 weeks'.
4. Payment schedule
Milestone-based payments tied to the programme. Not calendar-based. Not large upfront sums.
5. Variations procedure
How changes to the scope are instructed, priced, and agreed in writing before work proceeds.
6. Defects liability period
Typically 12 months. The contractor's obligation to return and fix defects that emerge after handover.
7. Defects retention
5% of the contract sum held back until the end of the defects liability period.
8. Insurance confirmation
Written confirmation that public liability, employers' liability, and contract works insurance are in place.
9. Subcontractor responsibility
The main contractor takes full responsibility for all subcontractors' work and insurance.
10. Building Regulations
Who is responsible for obtaining Building Regulations approval and managing the inspector's visits.
11. Dispute resolution
A clear process for resolving disputes — ideally adjudication under the JCT contract.
12. Practical completion
A defined process for handover, including a snagging list and the trigger for the defects liability period to start.
TCM uses the JCT Minor Works Building Contract as the base document for all residential projects over £10,000. Available from jctltd.co.uk for approximately £50. Worth every penny.
Payment Schedules: How They Should Work
The payment schedule is the single most important financial protection in a building contract. Get this right and you have leverage throughout the project. Get it wrong and you've paid for work that hasn't happened.
Payments should be tied to programme milestones, not calendar dates. Here's how TCM structures payment schedules for a typical house extension:
| Milestone | Payment | What It Covers |
|---|---|---|
| Contract signed | 10% deposit | Mobilisation, materials procurement, scaffolding |
| Foundations complete | 15% | Groundworks, concrete, drainage connections |
| Structural frame complete | 20% | Masonry, steelwork, structural engineer sign-off |
| Roof watertight | 20% | Roof structure, insulation, weatherproofing, glazing |
| First fix complete | 15% | Electrics, plumbing, internal studwork |
| Second fix and decoration | 15% | Plastering, flooring, kitchen/bathroom fitting |
| Practical completion | 5% (less retention) | Snagging complete, Building Regs sign-off |
| End of defects period (12 months) | 5% retention | Released when all defects resolved |
Why Clients Choose TCM
What We Offer — and What We'll Show You Before You Commit
TCM Building & Maintenance Ltd has been trading since 19 May 2014 (Companies House number 09044688). We carry £5 million public liability insurance, £10 million employers' liability insurance, and contract works insurance on every project. Our insurance certificates are available on request before any contract is signed — not after.
We use JCT Minor Works contracts as standard for all residential projects over £10,000. Our payment schedules are milestone-based, with a 5% defects retention released 12 months after practical completion. Our standard deposit is 10% of the contract sum. We don't ask for large upfront payments.
Across our last 63 completed residential projects in Hertfordshire and North London, the median contract sum was £49,600 for extensions and £43,800 for loft conversions. In 94% of those projects, the final account was within 5% of the original contract sum — the overruns that did occur were all client-instructed variations, not contractor surprises. That's the kind of financial predictability you should expect from a well-run building contract.
Our team includes structural engineers, architects, and interior designers who work together from initial feasibility through to final handover — which means you have one point of contact for the entire project, not a main contractor who disappears when the subcontractors arrive.
If you're currently comparing builders for a project in Hertfordshire, we're happy to walk you through our insurance certificates, our contract form, and our programme methodology before you make any commitment. That's the kind of transparency you should expect from any contractor you're considering.
Related Guides
Party Wall Agreement Guide
When you need one and what it costs
Building Regulations Guide
What each Part covers and how to get approval
House Extension Cost Guide 2026
Full cost breakdown with TCM's verified data
Should I Extend or Move?
The financial and lifestyle case for each
Party Wall Notices Explained
How to serve a notice, appoint a surveyor, and avoid disputes
Frequently Asked Questions
Ready to Talk to a Builder You Can Trust?
TCM Building & Maintenance Ltd has been trading since 2014. We'll send you our insurance certificates, our contract form, and a detailed fixed-price quote before you commit to anything.