Metricon Homes display, Aston Hills estate, Mount Barker — Building in Mount Barker

A single-storey extension typically costs $2,000–$3,500 per m². A 40m² addition runs $80,000–$140,000 including structural work, roofing, cladding, and fit-out. Two-storey additions cost more due to engineering and structural complexity. Always get a fixed-price contract rather than a cost-plus arrangement.

Yes in most cases. Extensions that change the building's footprint, height, or floor area require a building permit and often a planning permit. Some minor works fall under 'exempt development' thresholds — these vary by state and council. Your builder or building designer can advise what approvals apply to your project.

An architect designs the building and may project-manage, but doesn't do physical construction. A builder constructs the project to approved plans. A building designer (draftsperson) produces plans without an architect's full qualification. For most residential projects, a building designer plus licensed builder is sufficient and more cost-effective.

A standard prefabricated or modular granny flat takes 12–20 weeks from approval to completion. Traditionally-built granny flats take 16–26 weeks. The approval process (council or CDC) takes 4–10 weeks before construction can begin. Total time from concept to move-in is typically 6–12 months.

A compliant building contract must include: fixed price or schedule of rates, scope of works, start and completion dates, progress payment schedule, insurance details, the builder's licence number, variation process, and dispute resolution procedure. Never pay more than 10% deposit in most states.