Building Contracts for London Home Extensions: What You Need to Know

A building contract is the single most important document in a home extension project. It defines the relationship between you and your contractor, sets out what will be built, the price, the programme, the payment terms, and what happens if things go wrong. Many London homeowners proceed without a formal written contract, relying on quotes and emails — and many of the disputes and cost over-runs that result could have been avoided with a clear, signed contract before work began.

The Joint Contracts Tribunal (JCT) produces a range of standard contracts widely used in UK construction. For residential projects, the most relevant are the JCT Homeowner Contract (specifically designed for domestic building work where the contractor deals directly with a homeowner) and the JCT Minor Works Building Contract (suitable for smaller projects up to approximately two hundred thousand pounds). Both are widely available and used throughout the London residential market.

A contract for a home extension should at minimum include: the parties (your name and the contractor's name and company registration), a description of the works, reference to the drawings and specification documents that define the scope, the contract sum (the agreed price), the programme (start date, anticipated completion, and key milestone dates), the payment schedule (when interim and final payments are due), the retention percentage (typically three to five percent held back from each interim payment until completion), the defects liability period (typically six months after completion during which the contractor must return to rectify defects), the insurance obligations of each party, and what happens in the event of a dispute.

Variations — changes to the original scope during construction — must be managed through a formal change control process. The contract should specify that all variations are agreed in writing, priced before implementation, and recorded as a formal contract amendment. Verbal agreements on site are not enforceable and are the primary cause of post-completion cost disputes.

Payment terms matter. A well-structured contract has interim payments at defined milestones (for example, at completion of foundations, completion of structural works, completion of first fix, completion of plaster, completion of second fix, and final completion), with retention held on each payment. Never pay a large advance before work begins — a reputable contractor does not require significant upfront payment from an established client. A deposit of five to ten percent on signing is reasonable; beyond this, payments should follow completed work.

The defects liability period is your protection against defects discovered after practical completion. The retained amount (typically three to five percent of the contract sum) is held until the end of the defects liability period, giving the contractor an incentive to return and rectify defects. Insist on a formal defects liability period clause in the contract.

RCB issues formal written contracts on all extension and refurbishment projects in London as part of our professional principal contractor service. +44 7359 872594 | connect@rcbGroup.co.uk | www.rcbGroup.co.uk