What Contracts Do I Need for a Large Home Renovation or Extension?

Jonathan Lea February 23, 2026

Large renovation and extension projects are exciting, but they are also legally and financially complex. Many homeowners focus on design, budget, and timescales, only to discover later that the contracts they sign do not offer the protection they expected. Disputes over scope, fees, delays, and defective work are common, and they often arise not because anyone acted dishonestly, but because the contractual framework was unclear or poorly understood from the outset.

For substantial projects, particularly those involving architects, structural works, and significant spend, contracts are not just a formality. They are the primary mechanism by which risk is allocated, responsibilities are defined, and remedies are determined if something goes wrong. Having the right contracts in place is one of the most effective ways to manage that risk.

This guide explains the main architectural and building contracts typically used in large renovation or extension projects, how they work together, and why early legal advice can make a decisive difference.

Why contracts matter so much in large renovation projects

Smaller works are sometimes carried out on informal terms or short quotations. As projects increase in scale, relying on anything other than legally binding home renovation contracts becomes significantly more risky. That approach becomes increasingly risky as project size and complexity grow. Large renovations often involve multiple professionals, overlapping responsibilities, staged payments, and long timescales. Without a clear contractual structure, it can be difficult to establish who is responsible for what, and even harder to enforce your rights if problems arise.

Well-drafted contracts do more than set out price and scope. They provide mechanisms for change, deal with delay and disruption, allocate design responsibility, and set out what happens if the relationship breaks down. They also influence how easy, or difficult, it is to bring a claim or defend one.

From a legal perspective, many of the most expensive disputes could have been avoided, or at least contained, if the contractual position had been properly addressed at the start.

The architect’s appointment contract

What this contract covers

If you are appointing an architect, architectural technologist or other design professional, there should be a written appointment setting out the terms on which they provide their services. This is separate from the building contract and governs the design and professional input rather than the construction works themselves.

An architect’s appointment typically covers:

Scope of services: This should define precisely what the architect is responsible for, for example concept design, planning, technical drawings, contract administration, or site inspections. Ambiguity here often leads to disputes about whether something was included.
Fees and payment structure: Fees may be fixed, staged, or percentage-based. The contract should explain when fees become due and what happens if the project changes.
Design responsibility and limitations: This determines what the architect is legally responsible for if there is a design error, and whether any elements are excluded or passed to others.
Professional indemnity insurance: This is critical if something goes wrong. The contract should specify the level of cover and how long it will be maintained.

From a homeowner’s perspective, a common misconception is that an architect is automatically responsible for everything that goes wrong on site. In practice, liability depends heavily on what they were contractually engaged to do. In reality, their liability depends heavily on what they were contractually engaged to do.

The building contract with your contractor

The central contract for the works

The building contract is usually the most financially significant document on the project. It governs the relationship between you and the main contractor carrying out the works and sets out how the build will be delivered.

For large renovation projects, this should almost always be a formal written contract rather than an exchange of emails or a simple quotation. Commonly used standard forms include JCT contracts, often with amendments, although bespoke contracts are also sometimes used. For example, this may include the JCT Home Owner Contract or JCT Intermediate Contract, depending on the scale and complexity of the works.

A properly structured building contract will address:

Scope of works and specifications. This should tie back to the drawings and documents produced by the architect. Vague descriptions are a frequent source of conflict.
Price and payment mechanism. This may be a fixed price, staged payments, or a valuation-based system. The contract should clearly explain how and when payments are calculated.
Variations: Changes are almost inevitable in large projects, so the contract should set out how variations are instructed, priced, and agreed.
Time for completion and delays. This includes the completion date, extension of time provisions, and consequences of delay.
Defects and remedies: This covers what happens if work is defective, including defects liability periods, retentions, and any warranties provided for structural or specialist works.

Many homeowners assume that a standard form contract is automatically fair or safe. In practice, amendments can significantly alter the balance of risk, and these should always be reviewed carefully.

How the architect and building contracts interact

Understanding overlapping responsibilities

One of the most common areas of confusion arises from the interaction between the architect’s appointment and the building contract, and the coordination between design and construction contracts. These are separate legal relationships, and gaps or overlaps between them can create real problems.

For example, if there is a design defect that leads to defective construction, liability may depend on whether the issue arose from the architect’s drawings, the contractor’s workmanship, or a design element passed to the contractor. If the contracts are not aligned, you may find each party blaming the other.

From a legal standpoint, ensuring consistency between these contracts is crucial. This includes aligning definitions, responsibilities, and procedures for certification, variations, and dispute resolution. It should also include consistent use of terms such as “Practical Completion”, “Employer”, and “Contract Administrator”.

Key legal risks homeowners often overlook

Common problem areas

Large renovation disputes tend to follow familiar patterns. Understanding these risks in advance can help you avoid them.

Unclear scope and assumptions. If drawings or specifications are incomplete, contractors may price on assumptions that later become contentious. This often leads to expensive variations.
Inadequate variation procedures. Informal changes agreed on site can undermine your position if costs escalate or disputes arise.
Design responsibility gaps. Where responsibility for elements such as structural design or specialist systems is unclear, establishing liability later can be difficult.
Termination rights. Many homeowners only discover how hard it is to exit a contract when relationships have already deteriorated. Contractors may also seek to rely on heavily amended standard terms that shift risk unfairly onto the homeowner.

These issues are rarely resolved by relying on “common sense” alone. The contractual wording is usually decisive.

Time limits, claims, and dispute resolution

What happens if something goes wrong

If defects, delays, or cost overruns occur, your ability to take action will depend on the contracts in place. Limitation periods, notice requirements, and dispute resolution clauses all matter. Limitation periods, typically six years for simple contracts and twelve years where a contract is executed as a deed, determine how long you have to bring a claim.

Some contracts require disputes to be referred to adjudication before court proceedings can be issued. For example, under many JCT contracts, adjudication is usually the first step before court or arbitration. Others impose strict time limits for notifying claims. Missing these steps can significantly weaken your position.

Early legal advice can help you understand not only whether you have a claim, but how and when it must be pursued.

How early legal advice can save time, cost, and stress

Prevention rather than cure

From a commercial and practical perspective, the most effective time to seek legal advice is before contracts are signed. A relatively modest investment at that stage can prevent far more significant losses later. The key is seeking advice before any contract is signed.

A solicitor experienced in construction and residential development can:

Review and negotiate contracts. Identifying unfair risk allocation or unclear provisions before they become a problem.
Align appointments and building contracts. Ensuring responsibilities are clear and consistent across all documents.
Advise on project structure. Including payment mechanisms, security, and termination options.

This approach is not about creating unnecessary complexity. It is about ensuring that the legal framework matches the reality of the project you are undertaking.

How JLN can help

Large renovation and extension projects sit at the intersection of construction law, property law, and practical risk management. At Jonathan Lea Network, we advise homeowners on architectural appointments, building contracts, consultant terms, and related documentation for high-value residential projects.

In practice, clients typically come to us in one of two situations.

First, they already have a proposed contract in front of them – whether from a contractor, architect, or other consultant – and want it properly reviewed. We can analyse the terms, identify risk allocation issues, advise on exposure, negotiate amendments where appropriate, and refine or improve the contract to ensure it better protects your interests and reflects the reality of your project.

Alternatively, some clients prefer to put a comprehensive contractual framework in place from the outset. In those cases, we work with you to produce a carefully structured, bespoke contract designed specifically for your home construction project. This can cover the initial design stage as well as the build itself. We will discuss the key commercial and legal terms that should be included — such as scope, payment structure, design responsibility, delay provisions, variations, termination rights, and dispute resolution — and then draft a robust and clearly defined agreement tailored to your circumstances and objectives.

Our focus is always practical. We provide clear advice on risk, cost, and strategy, so you understand the implications of the contractual choices being made. Whether you need an existing contract reviewed and strengthened, or a well-considered and watertight agreement drafted from scratch, we aim to ensure that your project proceeds on a secure legal footing.

If you are planning a substantial renovation or extension, or have been asked to sign contractual documents and would like independent advice, we would be pleased to discuss how we can assist. Contact the Jonathan Lea Network team to arrange an initial consultation and ensure your project is properly protected from the outset.

Please email wewillhelp@jonathanlea.net or call us on 01444 708640 as a first step. Following an initial discussion, we can provide a clear scope of work, a fee estimate (or fixed fee where appropriate), and confirm any information or documentation we would need to review.

We usually offer a no-cost, no-obligation 20-minute introductory call as a starting point or, in some cases, if you would just like some initial advice and guidance, we will instead offer a one-hour fixed fee appointment (charged from £250 plus VAT depending on the complexity of the issues and seniority of the fee earner).

* VAT is charged at 20%

This article is intended for general information only, applies to the law at the time of publication, is not specific to the facts of your case and is not intended to be a replacement for legal advice. It is recommended that specific professional advice is sought before relying on any of the information given. © Jonathan Lea Limited.