Do You Need a Building Contract for a Home Extension?

This article is adapted from Chapter 4 of Your Seven Step Home Extension Plan by Ashton Paul — the book that shows homeowners how to manage a project and avoid the stresses and strains of the home extension process.

Whenever a homeowner phones us mid-crisis about a builder, Paul asks a few probing questions — and his heart drops when the usual answers come back. No formal contract. Little or no professional advice. From that position, the only guarantee going forward is large legal and professional fees for unravelling the mess.

So let's answer the question in the title properly. Yes, you need a building contract for a home extension — a real one, in writing. Here's what goes in it, and why.

"Verbal agreements aren't worth the paper they're written on"

Paul recently heard a client arguing with a builder that "the fencing costs were included in the £25,000 quote". There shouldn't be any dispute about a thing like that — it should be clearly itemised on the written quotation. Instead, the conversation went the familiar way: "No, we decided to take the fencing out because you were going to get another price, and I added the new rainwater gutters and downpipes, and reduced the price by £350."

Add this, omit that, tracking back over months of emails with no method of cost control. A total mess.

You may have an excellent memory. You can perhaps visualise exactly where you were standing when something was agreed. That's no use at all if your builder has a poor memory and didn't write it down. Which is why Paul's rule in the book is blunt: verbal agreements aren't worth the paper they are written on.

What a proper contract actually contains

The first requirement for any building contract is a good set of contract documents: the drawings, a specification or schedule of works, and the structural calculations. What most homeowners have instead is a verbal account and a pile of emails.

The key items the contract itself should record:

• The start and finish dates
• The parties — the employer (you), the contractor (the builder), and the contract administrator (your architect or surveyor, if you have one)
• The contract sum and payment terms
• The post-works retentions

Our recommendation, straight from the book's checklist: insist on a JCT building contract, with payments in arrears and retentions. Alongside it, establish a set of written rules with your builder, agree a day for weekly meetings, and clearly specify the scope of works — so that every builder pricing the job quotes on the same document. (We've covered how to choose the builder himself separately.)

Never vary the contract sum

This is one of the most practical tips in the whole book. The contract sum is a legally binding amount — understood, fixed, agreed. Don't ever change it.

Instead, run every change as a variation in a separate account, and apply the net total to the contract sum at the end of the job. The final account is then simply: contract sum + net variations + any additional items. Don't omit items from the contract sum either — it leads to confusion, and builders' final invoices are confusing enough already.

The final account is also a genuine opportunity, not just an administrative chore. Review the works, the variations and their costs. Builders are not very good at reporting omissions and cost savings — they tend to focus on the extras — so somebody has to check for the items that cost less than the specification allowed. It's nice to have some omits to balance those extras. Useful headings: "Additional Items" and "Adjustment of Provisional/Contingency Sums".

Penalties don't exist — but leverage does

Clients regularly ask whether building contracts have penalty clauses for delays. They don't. What they have are Liquidated and Ascertained Damages (LADs): if you incur real costs because of the builder's delay — and you didn't cause the problem — you can recover costs that were clarified in advance. The key word is real; LADs must be actual.

But here's Paul's experience from decades of contracts: a prompt or early completion bonus is likely to motivate your builder more than a penalty. Use both together — the carrot of the bonus and the stick of the LADs. Rely on the stick alone and the builder simply adds the cost of the penalty to your price as a risk premium, which frankly is what they do.

Communicating your own pressures works the same way. If you've cleared all your furniture into storage at £120 per week so the builder can work faster, tell him. He's then far less likely to pass on his own delay costs — and far more likely to gear up extra labour if the job slips behind. Say nothing, and he'll proceed at a pace that suits the works, not you.

Insurance: the claim you can't predict

Paul once had to manage a claim from a neighbour after a leak from a flat roof ran into her cupboard — apparently onto an antique Mulberry handbag. The builders carried £5 million of cover, so it wasn't a drama. The point of the story: you have no idea where insurance claims may come from.

Three checks before anyone starts:

1. The builder should hold Builders All Risk Insurance — covering breakages, personal accident and third-party liabilities — for the full duration of the works. Check all three covers exist.
2. The subcontractors — it doesn't hurt to ask about insurance for the electrics and the central heating installation specifically.
3. You — inform your own home insurer about the proposed works. And get good cover, not cheap cover: if your insurer says they won't cover you while building works are carried out, you have cheap insurance. Cover during works should be provided without additional cost — and if they refuse that, what else are you not covered for?

There's a sobering legal footnote here. In a landmark 2010 case, a London homeowner refused a builder access through her house to protect her carpets. Forced to use a ladder that was too short for the garage roof repair, he fell and suffered life-changing injuries — and because she had hired him directly, she was held liable for failing to provide reasonable access to the working area. He was cheap, and uninsured. Access arrangements and insurance aren't small print; they're the contract working exactly as intended.

Terms and conditions: set the ground rules

Like any good relationship, set the rules at the start — or don't complain when it goes wrong. Don't assume anything, even that the builders will turn up every day.

Your terms should cover your personal requirements: the standard of work you expect, how much mess you're prepared to endure, access restrictions to the rest of your home, even where vans can park. Quality in particular affects price directly — Paul prices three distinct standards, Standard, Higher and Best, each roughly £250 per square metre more than the last. If you want top quality, more preparation time and a different calibre of tradesmen are needed, and the price should say so before anyone starts.

Site tidiness matters more than you'd think, too. Paul once arrived at a colleague's self-managed project shortly before a council building control officer was due — and made the trades stop and spend an hour tidying the shambolic site first. A messy site is an instant red flag to a BCO: it suggests poor workmanship, which means more frequent inspections, stricter adherence checks, and ultimately increased costs — even when no corners are actually being cut.

Keep a project diary — and write the builder's letters for him

If your builder doesn't confirm costs and changes in writing, do it for him. A short confirmation covers the cost, the quality, the time implication and the practical details, and takes five minutes:

Dear Eddie — I write to confirm that I would like to add the cloakroom to the works at the additional cost of £1,450 plus VAT. I will supply the WC, cistern, seat, basin and taps; all other fitments to be provided by you. Please confirm any extra time needed. Two concerns: no exposed pipework, and please ensure your team don't use the new toilet, however tempting.

Beyond letters, keep a project diary — daily if you can, weekly at least. What was agreed, when, amounts, timescales; photos on your phone. One of Paul's clients turned hers into a blog. It can be genuinely cathartic — and if a disagreement ever occurs, a well-documented account gets a professional to the heart of the matter faster, which directly reduces your dispute-resolution costs. It would pay for itself many times over. (For works affecting a shared wall, the Party Wall Act brings its own paperwork — factor that in early.)

The paperwork checklist

From the end of Chapter 4:

• Establish a set of written rules with your builder
• Agree a day for weekly site meetings
• Inform your insurers about the proposed works
• Insist on a JCT building contract, with payments in arrears and retentions
• Clearly specify your scope of works — and make sure every builder quotes on the same document
• Keep a project file or diary
• Set and manage your own contingencies — and don't allow builders' contingencies

The contract is step five of Paul's seven-step process — it only works properly when the plan and specification come first, which is exactly how our design and build service runs every project. The full process, with checklists and template letters, is in Your Seven Step Home Extension Plan (£12.99).