How Much Does Building an Extension Cost?

Building an extension cost varies dramatically across Melbourne, with projects ranging from $50,000 for basic ground floor additions to over $300,000 for premium double storey extensions.

Multiple factors influence your final investment, from material selections and site conditions to council requirements and structural complexities. We at Cameron Construction see homeowners consistently underestimate hidden expenses that can add 15-25% to initial budgets.

What Drives Extension Costs in Melbourne

Extension pricing depends on four primary cost drivers that can double or triple your initial budget estimates. Size remains the most significant factor, with ground floor extensions typically costing $1,200 to $2,500 per square metre in Melbourne.

Double storey additions range from $2,000 to $4,000 per square metre due to structural complexities and scaffolding requirements. A basic 20-square-metre ground floor extension starts around $30,000, but bathrooms, kitchens, or premium finishes push costs toward $80,000 for the same space.

Material Selection Impact on Costs

Material choices create the widest cost variations in extension projects. Standard brick veneer construction costs approximately 30% less than rendered concrete block or natural stone façades. Timber floors add $80-150 per square metre compared to basic carpet at $30-50 per square metre. Kitchen and bathroom inclusions dramatically affect costs, with premium tapware and stone benchtops adding $15,000-25,000 to basic fit-outs. Homeowners consistently underestimate electrical and plumbing upgrades, which cost $8,000-12,000 for typical extensions that require new circuits and water supply connections.

Site Conditions and Access Challenges

Sloping blocks increase foundation costs by 25-40% compared to flat sites due to retaining wall requirements and complex footings. Narrow side access restricts machinery use, which forces manual handling that adds $5,000-15,000 to standard projects. Asbestos removal (common in pre-1980s Melbourne homes) costs $20-80 per square metre depending on material type. Rock excavation can add unexpected expenses of $150-300 per cubic metre when standard soil assumptions prove incorrect during construction.

Council Requirements and Professional Fees

Planning permits cost $1,500-3,500 across Melbourne councils, with heritage overlay properties requiring additional approvals that extend timeframes by 8-12 weeks. Building permits add another $2,000-4,000 depending on project value and complexity. Structural engineering reports cost $2,500-5,000 for double storey extensions, while building designer fees range from $8,000-15,000 for comprehensive documentation. Soil reports (mandatory for most extensions) cost $1,200-2,000 but prevent foundation issues that could add $20,000-50,000 in remediation costs.

These cost drivers vary significantly between different extension types, with ground floor additions offering the most predictable pricing structure compared to more complex multi-level projects.

What Does Each Extension Type Actually Cost?

Ground floor extensions deliver the most predictable costs in Melbourne’s construction market. Single-storey additions cost $2,200-3,500 per square metre, with basic 20-square-metre projects starting at $44,000 for simple spaces. Bathroom or kitchen inclusions push identical spaces to $60,000-80,000 due to plumbing, electrical, and fixture requirements.

Sloping sites add 25-40% to foundation costs, while heritage overlays require additional approvals that extend project timeframes by 8-12 weeks. Access restrictions on narrow blocks force manual material transport, which adds $5,000-15,000 to standard ground floor projects.

Double Storey Extension Complexities

Double storey extensions cost $2,000-4,000 per square metre due to structural requirements and scaffolding access. A 40-square-metre double storey addition ranges from $120,000-200,000 (depending on finishes and structural modifications to existing foundations). Structural reports cost $2,500-5,000 for load calculations and beam specifications.

Scaffolding adds $8,000-12,000 to project costs, while crane access for materials increases expenses by $3,000-5,000. Council permits for double storey work cost $3,000-4,500 compared to $2,000-3,000 for ground floor additions.

Second Storey Addition Requirements

Second storey additions require planning permits and building permits as required by law. Foundation upgrades cost $15,000-30,000 when existing footings cannot support additional loads. Temporary accommodation adds $15,000-25,000 for typical 12-16 week projects.

Roof removal and reconstruction costs $25,000-40,000 for standard tile or metal replacement. These projects range from $180,000-350,000 for complete second storey additions, with premium finishes and structural modifications pushing costs toward the higher range.

Council approval processes extend 12-20 weeks due to structural documentation requirements.

Beyond these visible costs, several hidden expenses catch Melbourne homeowners unprepared and can significantly impact your final budget.

What Hidden Costs Should You Budget For?

Extension projects in Melbourne consistently exceed initial budgets by 15-25% due to unexpected site conditions and professional requirements that homeowners overlook. Asbestos removal affects 60% of pre-1980s Melbourne homes and costs $20-80 per square metre (depending on material type and location). Rock excavation adds $150-300 per cubic metre when soil reports reveal unexpected conditions, while contaminated soil remediation can reach $50,000 for severely affected sites.

Structural modifications to existing foundations cost $15,000-30,000 when current footings cannot support additional loads. This issue particularly affects heritage homes built before modern standards.

Professional Fees Accumulate Rapidly

Building surveyors charge $2,500-4,500 for inspections throughout construction phases. Structural engineers require $2,500-5,000 for load calculations and certification. Project management fees range from 8-12% of total construction costs but prevent costly mistakes that typically cost three times more to rectify.

Insurance premiums increase 20-30% during construction periods. Temporary utility connections add $3,000-5,000 for power, water, and waste services. Legal fees for contract reviews cost $1,500-3,000 but protect against variations that average $15,000-25,000 in disputed additional work.

Temporary Accommodation Expenses

Kitchen and bathroom renovations require temporary accommodation that costs $200-350 per night for family-sized units. Construction periods typically last 8-16 weeks for major extensions. Storage fees for furniture and belongings add $150-300 monthly, while meal expenses increase by $100-200 weekly without functional facilities.

Childcare disruptions occur during noisy construction phases and cost additional $50-80 daily for alternative arrangements. This particularly affects families during school holidays when construction activity peaks in Melbourne’s calendar.

Site Access and Preparation Costs

Narrow side access restricts machinery use and forces manual material transport (which adds $5,000-15,000 to standard projects). Crane hire becomes necessary for materials delivery to upper levels and costs $3,000-5,000 per project. Tree removal and protection measures add $2,000-8,000 depending on council requirements and species protection orders.

Temporary fencing and site security cost $150-250 per week throughout construction periods. These expenses protect both workers and neighbouring properties from construction hazards.

Final Thoughts

Extension costs in Melbourne vary dramatically based on size, materials, site conditions, and council requirements that can triple initial estimates. Ground floor additions start at $1,200 per square metre, while double storey extensions reach $4,000 per square metre due to structural complexities. Hidden expenses (including asbestos removal, rock excavation, and temporary accommodation) add 15-25% to budgets.

Professional consultation prevents costly mistakes that typically cost three times more to rectify later. Structural engineers, builders, and surveyors identify site challenges before construction begins. Melbourne homeowners should obtain detailed quotes from licensed builders, budget for unexpected conditions, and engage professionals early in the process.

We at Cameron Construction specialise in double storey extensions, ground floor additions, and second storey conversions with in-house designers and project managers who navigate council requirements. Our team handles concept to completion including planning, permits, and BCA compliance for Melbourne homeowners. Contact Cameron Construction to discuss your extension project and receive expert guidance on building an extension cost for your specific requirements.

Disclaimer: This information is general in nature. Consult qualified professionals for advice specific to your circumstances.