Free HomeAdvice Tool

Home Extension Cost Calculator Australia

Estimate how much it could cost to extend your home based on extension size, project type, finish level, site conditions and major inclusions.

✓ Ground Floor Extensions ✓ Second Storey Additions ✓ Australian Cost Guide

1 Tell us about your extension Enter the approximate new floor area and select the type of extension you're considering.

New Floor Area (m²)

Extension Type

Finish Level

Site & Access

2 What's included in the extension? Add major rooms and project costs that could significantly affect the budget.

New Kitchen Cabinetry, benchtops, services and standard appliances New Bathroom Bathroom fit-out, plumbing, waterproofing and fixtures New Laundry Cabinetry, plumbing and basic laundry fit-out Large Glazing / Bifold Doors Allowance for large-format glazing or doors Existing Home Alterations Allow for additional internal renovation or integration work Design & Approval Allowance Preliminary allowance for design, engineering and approvals

Contingency 15%

Extensions involve an existing structure, so unforeseen structural, service or site work can occur. A contingency provides an additional planning buffer.

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Extension Costs

How Much Does a Home Extension Cost in Australia?

The cost of extending a home depends on much more than floor area. Building into an existing house, structural alterations, access, kitchens, bathrooms and the level of finish can substantially change the final project cost.

Ground Floor

Ground Floor Extension

$2,400 – $4,500/m²

Simple ground-floor extensions can start toward the lower end of this range, while standard projects with more complex integration and finishes can cost considerably more.

Second Storey

Second Storey Addition

$3,200 – $5,500+/m²

Building upward generally involves greater structural complexity, roof alterations, stairs and work to the existing building.

Premium

Architectural Extension

$4,500 – $7,000+/m²

Bespoke architecture, extensive glazing, premium materials and detailed integration with an existing home can push costs considerably higher.

Planning Your Project

What Affects Home Extension Costs?

Existing Structure

Extensions must integrate with an existing building. Structural alterations, demolition and reinforcement can add significant cost.

Kitchens & Bathrooms

Wet areas require cabinetry, plumbing, electrical work, waterproofing, fixtures and finishes, making them considerably more expensive than simple living areas.

Site Access

Tight access, sloping blocks and restricted machinery or material access can increase labour and construction costs.

Design & Finish

Architectural detailing, glazing, bespoke joinery and premium materials can significantly increase the cost per square metre.

About This Calculator

How the Home Extension Estimate Works

The HomeAdvice calculator multiplies the proposed new floor area by an indicative construction cost range based on the extension type and finish level. It then adjusts the estimate for site difficulty and major selected inclusions.

Extension pricing is particularly variable because the new construction must integrate with an existing building. Structural conditions, demolition, services, roof alterations, access and the condition of the existing house may not be fully understood until detailed investigation and design work has taken place.

Project Type Indicative Planning Range

Ground Floor — Basic / Simple $2,400 – $3,500/m²

Ground Floor — Standard $2,800 – $4,500/m²

Second Storey — Standard $3,800 – $5,500/m²

Premium / Architectural Up to $7,000+/m²

Please note: These figures are intended for preliminary planning only. They are not a quotation. Actual costs vary according to location, design, builder, existing conditions, engineering, approvals, access, specifications and market conditions.

Cost methodology reviewed September 2026.

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