Why Fixed Price Contracts Can Cost More Than Expected

A fixed price building contract is an agreement where the builder commits to completing your construction project for a set price, regardless of the actual costs they incur. This provides homeowners with cost certainty and predictability. However, it's important to understand that 'fixed price' doesn't mean completely unchangeable – certain circumstances outlined in the contract may still allow for additional costs, such as variations you request, unforeseen site conditions, or changes to materials specified as provisional sums or prime cost items.

Additional costs in fixed price contracts typically arise from five main sources: design and material changes you request after signing; unforeseen construction challenges like rock excavation or asbestos discovery; regulatory changes requiring compliance updates; significant supply chain disruptions affecting material costs (if allowed in your contract); and mistakes requiring rework. The ABB Power Generation case established that additional work performed with an expectation of payment may be recoverable, even without a direct contractual provision.

Quantum meruit is a legal principle meaning 'as much as deserved' – it allows a party to recover reasonable value for services or goods provided, even without a formal contract for that specific work. The ABB Power Generation case clarified that if your builder performs additional work with the expectation of payment, and you receive the benefit of that work, you may be liable for reasonable payment under quantum meruit principles. This is why clear documentation of all variations and additional work is essential to avoid disputes.

As a homeowner, you have several important rights: the right to receive clear, detailed information about scope, materials, timeline, and costs (including potential additional costs); the right to quality workmanship meeting contract standards and building codes; the right to a written variation process with detailed cost and timeline impacts; and the right to dispute resolution through mediation, arbitration, or legal action if problems arise. Understanding these rights before signing helps protect your interests throughout the construction project.

Variations are changes or additions to the work originally agreed in your building contract. They can range from minor alterations to significant project overhauls. Every variation should be documented in writing before work commences, with clear details about the change, the reason for it, the additional cost, and the impact on the project timeline. Proper variation management prevents disputes and ensures you understand exactly what additional costs you're agreeing to pay.

The ABB Power Generation Ltd v Chapple case established critical precedents for construction contracts: it confirmed that parties benefiting from additional work may be liable for payment even without direct contractual agreement; it established that additional work performed with an expectation of payment can lead to valid quantum meruit claims; and it emphasised the importance of clear communication and documentation of all project scope changes. This case has influenced how courts interpret obligations for additional work in construction disputes.

Absolutely. Pre-signature contract review by a specialist construction lawyer is one of the most cost-effective investments you can make. A professional review identifies unfair terms, clarifies your obligations and rights, explains provisions for additional costs (provisional sums, prime cost items, variation processes), highlights risks specific to your project, and ensures the contract complies with NSW Home Building Act 1989 and relevant building legislation. Early legal advice prevents expensive disputes and protects you from signing unfair terms that could cost thousands to challenge later.