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.

One advantage of using Duncan Thompson Extensions 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.

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

Free consultation

Duncan Thompson Extensions 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 Extensions 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.

Related Information

Home Extension Costs

One thing almost everyone wants to know is how much a home extension costs. This is a difficult question to answer, but here we explain what factors determine the cost of an extension.

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There are a number of factors outside of our control, however we’ve completed so many over the last 30 years that we have a pretty good idea how long it takes to build an extension.

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We offer fixed-price home extension contracts which include design, planning approvals and construction. The contract details everything, with costings, so there are no nasty surprises.

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Project Management

Your Building Supervisor will be onsite every step of the way to ensure everything runs smoothly. They are your day-to-day contact and will facilitate any requirements you may have.

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Building Inspections

During every home extension a series of inspections are carried out to ensure all aspects of the construction have been implemented safely and within mandatory building requirements.

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Our people, from the front office right through to the onsite trades have worked with us for many years. This continuity of people ensures a smooth workflow during every extension we build.

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Building Permits

We’ll determine what permits and planning approvals are required to build an extension to your home. We negotiate with Councils on your behalf so there’s nothing you need to do.

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Building Insurance

Every home owner should ensure they have adequate insurance on their home before they start an extension. We can assist you if you have concerns about your level of cover.

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