House extension contract example and payment schedule
An extension needs more definition than “build an extension to the plans”. The junction with the existing house, ground conditions, drainage and homeowner purchases can change both the cost and the payment plan.
Worked example only. These are fictional project details and amounts, not market rates or a completed agreement to sign. Adapt the scope and stages to the actual project, programme, cash flow and chosen contract.
Example contract arrangements
Project: Single-storey rear extension with an opening into the existing house, new structure and roof, service alterations and specified internal finishes. The fictional £120,000 contract sum includes any applicable VAT. It is an illustration, not an extension price estimate.
Work documents: Identify the current plans, structural information, specification, accepted quotation and payment schedule. State how conflicting documents will be resolved under the selected contract.
Groundworks and drainage: Describe what is allowed for foundations, drainage diversions, spoil removal and access. Identify assumptions and the process for agreeing genuinely unforeseen work.
Existing property and site setup: Allocate responsibility for temporary support and design, weather protection, dust/security arrangements, welfare, deliveries and protection of occupied rooms.
Kitchen and client purchases: Separate the builder’s work from kitchen supply, appliances, finishes and other client purchases. Record fitting, connections, storage and making good, rather than relying on the word “excluded”.
Programme and decisions: Identify the approvals and notices required before relevant work, the proposed sequence, long-lead items and dates for homeowner decisions. Record the price and programme effect of changes.
View an official contract sample
View JCT’s official Home Owner contract sample page (PDF). This shows the work-details section only; obtain the complete appropriate contract separately. Publisher copyright remains with JCT.
JCT’s Home Owner form for an appointed consultant provides for that person to administer the contract. Having an architect’s drawings, or buying Build Advocate’s remote review, does not by itself appoint a contract administrator. Agree that role and its fee separately where the chosen form requires it. Read the publisher’s guidance.
Worked payment schedule — £120,000
The amounts below add up to £120,000. They illustrate a structure, not recommended percentages. Define exactly what each stage includes and how the amount is assessed and becomes due.
Agreed mobilisation and procurement — £12,000
Identified setup and orders, including any long-lead items, supported by the agreed evidence and protection arrangements.
Paid to date: £12,000.Foundations and substructure — £18,000
Defined excavation, foundations, drainage and substructure scope is evidenced, with relevant inspection records.
Paid to date: £30,000.Walls and structural shell — £24,000
The agreed walls, structural openings and supporting work are in place; any necessary specialist checks are recorded.
Paid to date: £54,000.Roof, windows and weather-tightness — £18,000
The specified enclosure is complete, including the junction to the existing house and agreed weatherproofing.
Paid to date: £72,000.First fix and internal preparation — £18,000
The listed electrical, plumbing, heating and internal preparation work matches the specification.
Paid to date: £90,000.Second fix and specified finishes — £18,000
The agreed fittings, making good and finishes are complete, with exclusions and client-supplied items kept separate.
Paid to date: £108,000.Completion, records and final balance — £12,000
Use the contract’s completion and payment process; record defects, certificates, handover documents and agreed remaining items.
Paid to date: £120,000.
Before using a schedule like this
Match every amount to the actual programme, procurement needs and value of the agreed work. A pre-start payment needs a named purpose and evidence; it is not automatically a safe deposit. Record invoice dates, payment terms, how progress is assessed and any final-balance or retention arrangements in the chosen contract.
Use the contract’s process for payment, variations, delays and disputed items. An inspection date or a photograph alone does not settle everything payable. Legal terms and remedies need advice on the actual agreement and applicable jurisdiction.
Want someone on your side before you sign?
Build Advocate’s £999 Homeowner Project Review & Build Support reviews plans, up to three builders, pre-construction arrangements, programme and payments, with one comprehensive report, one consultation call and agreed milestone check-ins. This is independent construction advice, not a formal contract-administration or legal appointment.
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