How Much Does a Double Storey Extension Cost?

Double storey extension costs in Melbourne typically range from $3,500 to $6,000 per square metre, depending on design complexity and site conditions.

We at Cameron Construction understand that budget planning is the foundation of any successful extension project. Multiple factors influence your final investment, from soil conditions to council requirements.

This guide breaks down the real costs you’ll face when adding a second level to your Melbourne home.

What Do Double Storey Extensions Really Cost

Melbourne double storey extensions demand significant investment, with costs that range from $3,500 to $6,000 per square metre for quality construction. Premium finishes and complex architectural features push costs toward the upper end, while standard specifications with straightforward designs stay closer to $3,500 per square metre.

Site conditions heavily influence your budget. Homes that require underpinning or extensive structural work can add $50,000 to $100,000 to your project cost.

Council Permits and Compliance Expenses

Council permit fees range from $3,000 to $8,000 depending on your municipality, with additional costs for building surveyors and engineering reports. Soil testing is compulsory and costs $1,500 to $3,000, but prevents costly surprises during construction.

The Building and Plumbing Commission requires builders to provide domestic building insurance certificates for projects exceeding $16,000. This protection adds to your overall project cost but provides essential coverage.

Cost Comparison with Single Storey Extensions

Double storey extensions cost approximately 40% more per square metre than ground floor additions. Single storey extensions typically range from $2,500 to $4,200 per square metre, which makes them appear more affordable initially.

However, double storey additions deliver superior value by maximising land use without sacrificing outdoor space. The additional structural requirements (including reinforced foundations and stair construction) justify the premium. Double storey extensions also provide better return on investment, with Melbourne properties seeing value increases of 25% to 35% compared to 15% to 25% for single storey additions.

Hidden Costs That Impact Your Budget

Structural engineering reports cost $3,000 to $8,000 for double storey projects, while standard extensions need only basic assessments. Temporary accommodation during construction adds $15,000 to $25,000 for families who relocate during major structural work.

Utility relocations (including gas, electricity, and water services) commonly cost $5,000 to $12,000 extra. Heritage overlays in inner Melbourne suburbs can increase costs by 20% to 30% due to specialised materials and approval processes.

These factors separate realistic budgets from unrealistic expectations. Understanding these costs upfront helps you prepare accurate quotes and select the right builder for your project.

What Drives Your Extension Budget Higher

Your site conditions determine whether your double storey extension costs $350,000 or $500,000. Clay soil dominates Melbourne’s eastern suburbs and requires reactive soil classifications that demand special footings costing $15,000 to $30,000 extra. Rock formations in areas like Ivanhoe or Kew add $20,000 to $40,000 for excavation and specialised equipment. Sloping sites increase structural costs by 25% to 40% because builders must create level platforms and retaining walls before construction begins.

Soil Testing Reveals Hidden Expenses

Professional soil reports cost $2,500 to $4,000 but prevent budget blowouts that reach $80,000 when reactive clay expands during wet seasons. The foundation where the footing is to be located must be classified in accordance with AS 2870, with highly reactive sites requiring pier and beam foundations that double your footing costs. Sites with poor drainage need additional waterproofing and drainage systems costing $8,000 to $15,000. Tree root zones within 10 metres of construction add $5,000 to $12,000 for root barriers and specialised footings.

Design Complexity Creates Cost Escalation

Architectural features like cathedral ceilings, curved walls, or multiple roof lines increase construction costs by 30% to 50% over standard rectangular designs. Custom windows and doors add $15,000 to $40,000 to material costs, while standard specifications keep budgets controlled. Steel frame construction costs 20% more than timber but handles complex designs better and reduces long-term maintenance. Open-plan layouts with large spans require engineered beams costing $8,000 to $20,000 extra but create the spacious interiors Melbourne families want.

Council Requirements Add Unexpected Costs

Planning permits for double storey extensions cost $3,000 to $8,000 depending on your municipality (with fees calculated at $16.81 per fee unit for 2025-2026). Heritage overlays in inner Melbourne suburbs increase costs by 20% to 30% due to specialised materials and approval processes. Building surveyors charge $2,500 to $5,000 for permit applications, while structural engineers add $3,000 to $8,000 for complex designs. These professional fees accumulate quickly but protect you from costly compliance issues later.

Understanding these cost drivers helps you prepare realistic quotes when you approach builders for your project. Melbourne extension projects experience cost increases that average 15-20% from initial estimates, so allocating an additional 10-20% buffer prevents financial stress during construction.

How to Get Builder Quotes That Protect Your Investment

Melbourne homeowners receive extension quotes that vary by $100,000 for identical projects, which makes thorough evaluation essential for your investment protection. We at Cameron Construction see homeowners accept quotes without understanding what drives these price differences or recognising warning signs that lead to cost blowouts.

Essential Questions for Builder Evaluation

Smart homeowners ask builders to itemise structural work separately from finishes and request breakdowns for excavation and underpinning costs. Demand specific timelines for permit applications and construction phases from every builder you interview. Ask builders about their experience with reactive clay soil conditions in Melbourne’s eastern suburbs, their approach to heritage overlay requirements, and their process for managing cost Variations during construction. Builders who cannot provide detailed answers to these questions lack the expertise your double storey extension requires.

Payment Schedules That Protect Homeowners

Do not pay your deposit until the builder has given you a domestic building insurance policy and certificate of currency for your property. Legitimate builders request 5-10% deposits maximum, with subsequent payments tied to foundation completion, frame completion, lockup stage, and practical completion. Avoid builders who demand 20-30% upfront payments or request full material costs before construction begins.

Payment schedules should align with Building and Plumbing Commission guidelines, which protect homeowners through domestic building insurance requirements for projects exceeding $16,000. Reputable builders provide detailed milestone descriptions that specify exactly what work must be completed before each payment becomes due.

Red Flags in Extension Quotes

Quotes under $3,200 per square metre for double storey extensions indicate corners will be cut through substandard materials or Unlicensed subcontractors. Builders who provide verbal estimates or refuse to detail permit costs, soil testing requirements, or structural engineering fees create budget uncertainty that costs homeowners thousands later.

Prime cost allowances below $15,000 for kitchens or $8,000 for bathrooms in double storey extensions signal unrealistic pricing that leads to expensive Variations. Legitimate quotes include specific timelines for council approvals (8-12 weeks), construction duration (16-24 weeks for standard projects), and clear Variation procedures that protect both parties from scope creep.

Timeline Expectations for Melbourne Extensions

Double storey extension projects require 6-8 months from contract signing to completion, with permit approvals consuming 8-12 weeks of this timeline. Builders who promise faster completion times often compromise quality or lack proper council relationships. Weather delays add 2-4 weeks during Melbourne’s winter months, while heritage overlay approvals extend timelines by an additional 4-6 weeks in inner suburbs.

Compare at least three written quotes from registered builders to assess value and expectations clearly. Consider exploring home improvement financing options to manage your extension investment effectively.

Final Thoughts

Double storey extension cost planning demands thorough understanding of Melbourne’s Unique challenges, from reactive clay soils to heritage overlays that add $50,000 to your budget. Your investment of $3,500 to $6,000 per square metre delivers exceptional returns when you manage it properly. Poor planning creates expensive surprises that derail projects and strain family finances.

Experienced extension builders prevent the cost blowouts that affect 60% of Melbourne renovation projects. Builders who understand local soil conditions, council requirements, and structural complexities protect your investment through accurate quotes and professional project management. They handle permit applications, structural engineering, and construction phases with expertise that saves you time and money.

Your next step involves obtaining three detailed quotes from registered builders who specialise in double storey work (verify their Building and Plumbing Commission registration first). Review their recent projects and confirm their domestic building insurance coverage before you sign contracts. We at Cameron Construction deliver double storey extensions across Melbourne, handling everything from concept design to BCA compliance with our in-house team that manages permits, engineering, and construction to complete projects on time and within budget.