Is a fixed-price contract or cost-plus better for your project?

The two contract types, in plain terms

Fixed-Price Contract — a lump sum is agreed at the start of the project, covering all labour, materials, and services. In theory, this gives you cost certainty upfront.

Cost-Plus Contract — you pay the actual cost of materials and labour, plus a fixed builder's margin. This keeps everything transparent and flexible, adapting as the project evolves.

When fixed-price works best

A fixed-price contract can be a good choice when the plans are mostly complete and contain a solid level of detail. The more information provided upfront, the more accurate the price will be — reducing the risk of provisional sums or unexpected variations throughout the project.

For example, if an Interior and Exterior Item Schedule is supplied at quoting stage, that may include (interior items first, then exterior):

• —Plumbing and electrical PC items
• —Electrical and air-conditioning plans
• —Cabinetry plans
• —Nib walls, bench seats, study nooks
• —Stairs, balustrades, and handrails
• —Internal wall linings
• —Feature walls
• —Architraves and skirting boards
• —Cornices
• —Square set finishes
• —Doors, hinges, and hardware
• —Thresholds
• —Tiling
• —Tiling heights and locations
• —In-wall shelving and shower niches
• —Floor wastes
• —Flooring
• —Paint types and colours
• —Fireplaces
• —Blinds and shutters
• —Bricks and mortar
• —Concrete slab finish
• —Windows and doors
• —Window and door hardware
• —Garage doors
• —Exposed frame or post types
• —Roofing, guttering, and downpipes
• —Skylights
• —Solar panels
• —Cladding
• —Rendering
• —Architraves, fascia boards, eave moulds, and trims
• —External ceiling linings and joining details
• —Decking, tiling, and screening
• —External stairs
• —Balustrade
• —Retaining walls
• —Awnings, carports, and patios
• —Plumbing and electrical service plans and PC items
• —External paint types and colours
• —Existing service location plans
• —New drainage details
• —Driveways and external paving
• —Landscaping plans

Advantages of fixed-price contracts

When everything is finalised upfront — often more achievable in new builds where we're starting from scratch — a fixed price can work well.

• —Budget Visibility – a clear contract sum is provided before work starts, based on the available plans and selections
• —Helpful for Finance Applications – lenders often prefer fixed-price contracts because they can see a defined contract amount
• —Suited to Predictable Projects – works well for straightforward new builds or projects where plans and specifications are detailed and unlikely to change
• —Less Ongoing Involvement – because the main pricing is agreed upfront, there's less need for the client to review costs as frequently during construction

A note on renovations and extensions

With renovations and extensions, unforeseen conditions in existing structures often mean the “fixed” price can change through variations. This doesn't mean fixed-price is wrong — it just means it works best where unknowns are minimal, and project details are already well defined.

When cost-plus works best

A cost-plus contract can be a great choice when flexibility and transparency are important. Under this arrangement, you pay the actual cost of materials, subcontractors, and labour, plus our agreed margin. This structure allows for adjustments as the project progresses, helping you keep control over selections, finishes, and budget decisions along the way.

This is particularly suited to renovations and extensions, where hidden conditions and evolving decisions are common. Variations may still occur (e.g. scope changes or late selections), but they're far easier to manage without blowing the budget.

Advantages of cost-plus:

• —Flexibility – choose finishes and fixtures during the build, instead of locking everything in months earlier
• —Control – adjust selections as the budget unfolds
• —Transparency – every invoice and receipt is tracked in our system, accessible to you at any time
• —Adaptability – hidden issues or client changes are handled without inflated variation fees
• —Collaboration – encourages open communication and a true team approach

How we manage cost-plus projects

We've refined a system that keeps you fully informed and in control:

• —Detailed budget and estimated cost breakdown prepared before work begins
• —Fortnightly budget updates using our cost-tracking spreadsheet, showing suppliers, costs to date, and forecast
• —Full transparency with every invoice and receipt recorded and available to you

A real-world example

Imagine a builder quoting a fixed price for an extension. They don't know if material prices will rise, or if the existing structure has underlying issues. For example, walls may not be square, plumb, level, braced, or structurally reliable. To protect themselves, the builder adds a large buffer to the fixed price. If those issues never come up, the client still pays for them.

With cost-plus, you only pay for what actually happens. No hidden contingencies, no inflated “just in case” allowances — just the true cost of your project, managed openly and fairly.

Our recommendation

Both contract types have their place. If you have a good level of detail finalised before starting, fixed price may work for you. But in our experience, cost-plus offers the fairest, most transparent path forward — particularly for renovations and extensions. It avoids inflated pricing, keeps decision-making flexible, and builds a collaborative partnership between client and builder. That's why it's our preferred model.