Preparing for a Cost-Plus Building Project

What Is a Cost-Plus Contract?

A cost-plus contract is a building agreement where you pay the actual cost of materials and labour, plus a transparent builder's margin. Unlike a fixed-price contract — where the builder quotes a single figure upfront — a cost-plus arrangement separates the real construction costs from the builder's fee, giving you full visibility into where your money goes.

This model is particularly well-suited to renovation projects, where existing conditions can be unpredictable and client selections often evolve during the build. It provides flexibility to adjust scope, upgrade materials or change details — with any variations properly documented in writing and signed as required by NSW regulations.

How the Cost-Plus Model Works

The cost-plus structure typically breaks down into two clear components:

• Direct costs — the actual cost of materials, trade labour, equipment hire, permits and fees. You receive copies of supplier invoices and trade quotes so you can see exactly what has been spent.
• Builder's margin — a pre-agreed percentage applied to direct costs. This covers the builder's overheads, supervision, project management, site coordination and profit. Because it's expressed as a percentage, the builder's incentive can be aligned with delivering a quality result rather than cutting corners to protect a fixed margin.

Benefits of the Cost-Plus Approach

• Transparency — cost-plus progress claims should be supported by invoices, receipts or other verifying documents, helping the homeowner understand the actual project costs and the builder's agreed margin.
• Flexibility — client-requested changes can be accommodated, but any variation to the agreed scope, plans or specifications must be documented correctly. In NSW, variations generally need to be recorded in writing and signed by both the homeowner and contractor before the varied work begins, except in limited urgent circumstances. The variation should explain its effect on the project cost and completion time.
• No padding for unknowns — in a fixed-price contract, builders may add contingency to cover unforeseen conditions. With cost-plus, you generally pay for work as it is performed, which can reduce the need for up-front contingency allowances.
• Quality focus — the builder isn't incentivised to use the cheapest materials to protect their margin. You make informed decisions about where to invest in quality.

NSW Contract and Variation Requirements

In NSW, residential building work generally requires a written contract when the contract price is more than $5,000 including GST, or when the price is unknown but the reasonable market cost of labour and materials is more than $5,000 including GST.

For larger residential projects, the contract must explain when the final price is not known or may be subject to change. A cost-plus arrangement does not remove the requirement to properly document the agreed work, payment method, variations and progress claims.

Any agreement to vary the contract, plans or specifications generally needs to be recorded in writing and signed by both the homeowner and contractor. The written variation should describe the change and explain its effect on the overall project price and completion time before the varied work begins, except in limited urgent circumstances.

Progress claims for cost-plus work should correspond with work performed and costs incurred. They should be supported by invoices, receipts or other records that allow the homeowner to verify the claimed costs and the application of the agreed builder's margin.

Homeowners should read and understand their proposed building contract and obtain independent legal advice where appropriate before signing or approving significant variations.

For current NSW residential building contract requirements, visit NSW Government — Contracts for Residential Building Work.

How to Prepare for a Cost-Plus Build

To get the most out of a cost-plus arrangement:

• Start with a detailed estimate — your builder should provide a comprehensive estimate breaking down expected costs by category (demolition, framing, plumbing, electrical, finishes, etc.). This gives you a realistic budget framework.
• Make selections early — choosing tiles, tapware, appliances, flooring and paint colours before construction starts reduces delays and helps your builder order materials at the best price.
• Maintain a contingency — set aside 10-15% of your estimated budget for variations, unexpected conditions or upgraded selections.
• Review costs regularly — your builder should provide regular cost-to-date updates so you can track spending against the estimate throughout the project.
• Keep communication open — the best cost-plus relationships are built on trust and regular conversation. Ask questions, review invoices, and stay engaged with your project's progress.

ACG Built's Cost-Plus Approach

At ACG Built, we work on a clear estimate and cost-plus basis for all our renovation, extension and new build projects across the Sutherland Shire. Led by Austin George (Contractor Licence No. 347521C), we provide detailed initial estimates, regular cost tracking updates, and complete transparency around materials, trade costs and selections. This approach ensures you have control over your budget while we focus on delivering a quality build.

To discuss your project and receive a free, no-obligation estimate, contact ACG Built today.