House Extension Costs

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How much does a home extension cost?

The only way to truly calculate the cost of a home extension is to combine the costs of design, building materials, demolition, construction and labour. There are a number of other factors that can influence the cost also, like the age and condition of the house, the foundations or even the soil.

This is why we recommend to anyone that the best way to find out the costs for you to extend your home is to talk to us. We are sometimes even be able to give you a guide price over the phone.

Impact that various factors can have on the overall cost of an extension


  • The age of house, matching existing features, etc.
  • Condition of foundations and soil type
  • Renovation content required: e.g. repositioning of existing plumbing, etc.
  • Engineering content: e.g. unsupported corners on an upper storey


  • The size of your extension (sqm)
  • Your choice of materials
  • Fittings and finishings
  • Design revisions after construction has begun
  • Access to your block


  • Council regulations and permits
  • The style of your house
  • The amount of windows and exterior doors
  • Whether you continue to live in the house during building

The majority of these issues are addressed during the design stage with our experienced Designers and are usually not a major concern in most projects.

Free consultation

Duncan Thompson offers a free, no obligation consultation. When we meet with you we will develop a brief that will include your budget and what you expect to get for the money you spend. Because Duncan Thompson is a full-service, concept-to-completion home extension specialist, we are confident the guide price we give you will be close to the real cost to design and build your new extension.

Fixed-price guarantee

Once we have the necessary information from you and the building contact has been signed, all known costs are included in the price. If you change your mind once the build commences, we reserve the right to revisit the overall price quoted, however if this occurs you will be involved and informed of any additional costs before they happen.

One advantage of using Duncan Thompson to design and build your extension is that you will know how much your extension will cost, right from the beginning of the process, with no nasty surprises or extras.

House Extension Enquiries

Extension details

Getting started

Jim Despotellis Design Manager

Planning your home extension doesn't have to be daunting. Let us take some of the stress away. We offer a free, no obligation design consultation.

A senior member of our design team will be happy to talk to you and arrange a design consultation. We'll listen to your needs and discuss the best options for your family's lifestyle and budget.

Phone us on (03) 9836 8655 today to arrange your obligation-free Design Consultation across Melbourne's inner Eastern and bayside suburbs including Camberwell, Glen Iris, Brighton East, Surrey Hills, Balwyn, Hawthorn East, Brighton, Kew, Balwyn North, Caufield North and others!