JCT Contract Types for Homeowners: Which Contract Should You Use?

Introduction

When you appoint a builder to construct your rear extension or loft conversion, you are entering a legally binding contract. The terms of that contract determine what happens when the builder is late, when work is defective, when unforeseen costs arise, or when a dispute occurs. Most homeowners in NW3 do not give this contract sufficient thought at the outset — and some discover only after problems arise that the informal letter or email they exchanged with their contractor provides very little protection. The Joint Contracts Tribunal (JCT) publishes a suite of construction contracts used throughout the industry. For domestic renovation projects, one or two of these contracts are particularly relevant. This guide explains the main options, what they cover and when your architect should administer the contract on your behalf. For context on procurement choices, see our procurement options guide.

Why Construction Contracts Matter

A construction contract sets out the rights and obligations of both parties — the employer (you, the homeowner) and the contractor (the builder). A well-drafted contract provides:

• A fixed or defined price for the works, with clear provisions for how variations are valued
• A completion date, with provisions for extensions of time and delay damages
• A mechanism for making interim payments during the works
• Provisions for inspecting and approving (or rejecting) work during construction
• A retention mechanism — a percentage of the contract sum withheld until defects are remedied
• A dispute resolution mechanism — adjudication, arbitration or litigation — if agreement cannot be reached
• Provisions covering insurance, contractor insolvency and termination

Without a formal contract, you are dependent on the general law of contract and tort — which provides much weaker protection and is far more expensive to enforce.

The Main JCT Contracts for Domestic Projects

JCT Minor Works Building Contract (MW)

The most commonly used JCT contract for domestic extension and renovation projects in NW3. It is designed for straightforward projects where an architect or contract administrator is appointed and where the employer (homeowner) provides the design. Key features:

• Simple structure, relatively short document (approximately 50 pages)
• Fixed contract sum, adjusted for variations
• Architect issues instructions, certificates and practical completion certificate
• Retention of 3–5% of contract sum withheld until defects rectification period expires
• Appropriate for contracts up to approximately £500,000
• Requires an architect or contract administrator to operate — it is not a self-administered contract

The JCT MW is the right choice for most NW3 single-storey rear extensions, loft conversions and moderate whole-house refurbishments where the architect is providing a full service including contract administration.

JCT Homeowner Contract

Specifically designed for domestic works where the homeowner does not have an architect acting as contract administrator. A much simpler, plain-language document that homeowners can understand and operate themselves. Key features:

• Short (approximately 12 pages) and written in plain English
• No requirement for an architect to administer
• Fixed price with a schedule of works
• Limited provisions for variations — must be agreed in writing
• Appropriate for simpler projects where professional contract administration is not being provided

The JCT Homeowner Contract is appropriate for smaller projects — a bathroom refit, a kitchen installation, a simple rear extension where the homeowner is managing the project directly without architect involvement during construction. It provides much better protection than an informal agreement but does not offer the full protections of the MW.

JCT Intermediate Building Contract (IBC)

Designed for larger projects (typically £250,000–£2 million) where there is a more complex programme and the employer requires greater control over the works. In domestic terms, this is appropriate for large whole-house refurbishments, basement projects or projects involving specialist subcontractors. The IBC is more complex than the MW — administering it requires a confident architect and a contractor familiar with its provisions.

JCT Design and Build Contract (DB)

Used when the contractor takes responsibility for design as well as construction — the contractor employs their own design team rather than building to the architect's drawings. This is uncommon in domestic renovation projects in NW3 where homeowners typically want an independent architect to protect their interests. For most domestic extensions and loft conversions, a design-and-build arrangement is not recommended.

Key Contract Provisions to Understand

Contract Sum and Variations

The contract sum is the agreed price for the works as described in the contract drawings and specification. Any change to the scope of works — additional work requested by the client, unforeseen conditions on site, corrections to errors in the design — is a variation. Variations must be instructed in writing by the architect and valued before work proceeds. Without this discipline, costs can spiral without warning.

Retention

Retention is a percentage (typically 3–5%) of the contract sum withheld from each interim payment. Half the retention is released at practical completion (when the building is finished and ready for use); the remaining half is released after the defects liability period (typically 6–12 months after practical completion). Retention protects the client against defective work that appears after completion.

Practical Completion

Practical completion is the point at which the works are substantially complete — the building can be used for its intended purpose, even if minor defects and snagging items remain. It is certified by the architect. After practical completion, risk in the building passes to the employer, half the retention is released, and the defects liability period begins.

Defects Liability Period

During the defects liability period (6–12 months in the JCT MW), the contractor is obliged to return and rectify any defects that appear and that are attributable to their workmanship or materials. At the end of the period, the architect issues a final certificate and the remaining retention is released. See our handover and defects checklist for what to inspect at practical completion.

When Your Architect Acts as Contract Administrator

Under the JCT MW, your architect acts as a neutral contract administrator — they are not simply your agent. They have duties to both parties: to instruct the contractor correctly, to certify payments fairly, and to resolve disputes impartially. The architect issues:

• Instructions (for variations, nominated subcontractors, opening up for inspection)
• Interim payment certificates (typically monthly)
• Practical completion certificate
• Non-completion certificate (if the contractor fails to complete by the contractual date)
• Certificate of making good defects
• Final certificate

The contract administrator role is included in a full architectural service. If your architect is providing planning and design services only and you are managing construction yourself, agree in advance whether contract administration is included — it is a significant additional responsibility. See our construction stage coordination guide and architect appointment terms guide for more on the full service.

Conclusion

Using a properly drafted JCT contract for your building project — even a relatively modest extension — is one of the most effective ways to protect yourself as a homeowner. The JCT Minor Works contract administered by your architect provides a framework for managing the construction stage professionally: clear pricing, payment control, retention, defects protection and a defined route to dispute resolution. The alternative — an informal arrangement without a formal contract — leaves you significantly exposed if problems arise. Discuss contract choice with your architect at the procurement stage, before you appoint a contractor. Use our free matching service to find an architect in NW3 who provides full contract administration as part of their service.