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Cost of a house extension

Last updated: 8th May 2019

House extensions can take many forms: an additional story, extending a living space out to the backyard or creating new rooms that increase the size of the internal property area. Accordingly, square metre costs can vary considerably on a range of factors from the need and extent of structural changes to the property, custom engineering analysis such re-configuring load bearing walls, the cost of custom materials such as large windows and even the location of your property.

According to quotes submitted during 2017-2018 on, the average cost of a house extension job was approximately $44,000. Building rates for extensions range from $2,000 – $3,000 per square metre for straightforward renovations on a level single floor, while more complex extensions and those involving multiple stories range from upwards of $4,000 per square metre as they involve additional associated costs such as greater compliance requirements, structural engineering and higher grade materials.

Example House Extension Jobs Posted on

Nomer from NSW: Double story extension
Council Approval: They have council approval
Plans: No – they need an architect
Materials: I need materials sourced for the job
Requirements: We are seeking a builder for a double story extension to a single fronted property. The extension is approximately 34sq metres (both floors inclusive). We also require additional work to be completed on the remaining section of the house. We have rear lane access to the property. We are seeking a builder who can provide us with references for similar projects that have been completed. We are hoping to start at the end of January 2014. We can send you floor plans through email as well as a list of other jobs to be done on the existing house and site inspections are recommended.

Total Cost: $220,000

Donna from SA: 1950’s Brick House Extension
Council Approval: They need help getting council approval
Plans: No – they need an architect
Materials: I need materials sourced for the job
Requirements: 1950’s house needs extension. Under roof garage, kitchen, living area, bedroom. Conversion of current kitchen to bathroom and ensuite

Total Cost: $250,000

James from Canberra: House extension with some renovaiton work required
Council Approval: They need help getting council approval
Plans: No – they need an architect
Materials: I need materials sourced for the job
Requirements: Need a registered builder for a house extension (with a little bit renovation) in Mt Waverley area
Extension part (total new area will be apox 130 SQM)
(build on stump footing with weather board outside and metal roofing would be great)
1.adding a new family home (36-38 SQM)
2.adding a new home office/home studio (42-45 SQM)
3.adding a new laundry (8 SQM)
4.adding a new ensuite (8 SQM)
5.adding a small storage room(8 SQM)
6.adding a new carport (15 SQM)
7.building a wooden fence that is 13m long,1.6m high,and install an auto gate(3.5m long.1.6m high)
Renovation task:
1.Enlarge the existing master bedroom by 30%. (from 14.1 SQM-20.2SQM)
2.demolish the old pergola and build a new one with roof cover (31 SQM)
3.demolish the old garage (25.45 SQM)
Applicants must be registered builders and must provide builder’s warranty,insurance,etc

Total Cost: $200,000

Cost of a single story extension

Building costs for a residential ground floor extension typically range from $2,000 – $3,000 per quare metre. Therefore a typical extension of 60 sqaure metres could cost around $150,000. However ground work costs such as foundations can cause your cost to increase considerably if there are complications with your build. Because of this, it is highly recommended to have qualified builders and earthworks professionals provide a thorough site assessment before work commences.

Cost of a second story extension

Building costs for a residential second story extension typically range from $4,000 – $5,000 per quare metre. Therefore a typical double story extension of 60 square metres could cost around $270,000. Additional costs for second story extensions arise from structural work required to support additional building weight and engineering compliance. Council approval may also be more difficult as height restrictions may apply, you will likely be significantly changing the aesthetic profile of the building, may block out light or overlook into neighbouring properties.

Cost of extension designing and planning

You may choose to either hire an architect for a complete full service design and plan of your home extension build, or do the design yourself if you are comfortable with building requirements and have a strong understanding of residential design.

If designing yourself, you will still need the services of a draftsman to draw up your designs into formal architectural drawings that can be used by your chosen builder.

Council approval for extensions

You will always require the approval of your local council for residential extensions because these types of renovations are often structural and/or change the internal footprint of your home. Every council has different restrictions, however common council requirements for home extensions include:

  • Minimum boundary gap: your extension cannot be within a minimum distance to neighbouring and street frontage boundaries. This is primarily to leave a fire safety gap across neighbouring properties
  • Height restrictions: particularly to prevent blocking out light or overlooking into neighbouring properties
  • Dwelling character restrictions: these can be particularly restrictive for homes on heritage sensitive streets, or the property itself could be heritage listed which may prevent any sort of extension at all, or limit you to only extending the back of the house on a single level in order to preseve existing street frontage design

Although the cost of council approval and inspections will be relatively small compared to the total cost of the extension project, they can add significant delays and cause secondary costs by creating bottlenecks in the building process if certain conditions are being processed after the build has already commenced. It is very important to confirm your intentions and plans with the council first to understand potential areas where your building plans could conflict with their requirements, before you commence building.