Ask each builder to price the same current brief
Use one drawing revision, one specification and the same owner requirements. Include the bathroom arrangement, laundry, appliances, heating, storage and finishes you expect. If one quote prices a different layout, identify that difference before debating its total.
Keep a quote register showing the date, validity period, drawing revision and exclusions. When the brief changes, ask for a revised quote or a clearly identified adjustment. A later email should not silently replace the scope attached to an earlier number.
Build a responsibility matrix for the whole project
For each work item, record whether it is included, an allowance, excluded, owner-supplied or awaiting information. Include demolition, foundation preparation, service trenching, switchboard work, paths, fences, landscaping, waste removal and reinstatement where relevant. Blank cells are unresolved scope, not free work.
Specify the starting and finishing point of each item. “Sewer connection” can leave open the route, tie-in, authority process and reinstatement. “Site access” can leave open whether fences are removed and who restores them. The matrix should identify the party completing each interface.
Separate a selected item from an uncertain quantity
Consumer Affairs Victoria distinguishes prime cost items, involving selections not finally identified and costed, from provisional sums for work whose cost is still estimated. Ask what each allowance covers and what evidence will make it final.
Keep your comparison on the same basis: item, allowance, assumed quantity, excluded components and adjustment method. Do not simply add an arbitrary contingency to one builder and leave another unchanged. Track the specific unresolved items so the budget can improve as the evidence improves.
Make the price basis and completion point visible
Check whether each quoted amount includes GST and whether costs paid directly by the owner sit outside it. Identify which inspections, documents, connections and practical tasks remain before the dwelling can be used as intended. The relevant permit determines the completion documentation required.
Separate the building quote from the project budget. Adviser fees, owner works, financing costs and operating setup may have different suppliers and dates. A quote can be internally accurate while still representing only part of the cash commitment.
Compare how unresolved items will be closed
Ask who obtains missing information, when a decision is required and how a change affects the scope, price and programme. A clear route to closing uncertainty is useful evidence when comparing offers; an unsupported promise that nothing will change is not.
For a proposed variation, keep the reason, current and revised scope, cost and timing together. CAV provides guidance on written changes to major domestic building contracts. Get the project-specific terms reviewed before relying on an informal message as authority to proceed.
Documents to take to the next conversation
- The same drawing revision and specification attached to every quote.
- An item-by-item inclusion, allowance, exclusion and responsibility matrix.
- GST basis, owner-paid costs and the intended completion point.
- Open questions with evidence required, owner and due date.
Questions owners ask
Is the lowest granny flat quote necessarily the cheapest project?
The total alone cannot answer that. Compare the same design, included site and service work, allowances, owner-paid items and completion point.
How should I compare provisional sums?
Compare what each sum covers, the quantity and site assumptions, excluded components and adjustment method. Identify which record will resolve the uncertainty.