How to Calculate the Cost of Building an Extension

Calculate the costs of building an extension prior to construction.

Building an extension to your home is a costly project that can easily lead to budget-busting overruns. To avoid this, calculating an accurate cost for the extension beforehand is vital. This is especially true for a DIY builder intending to save money by cutting out the labor and contractor costs. To make that accurate calculation requires some research on your part beforehand. You'll need to know everything that goes into creating the extension. Once you've done the research and priced the materials needed, making the calculation is an easy process that will save you frustration in the long run as it saves your budget along the way.

Step 1

Create a building plan that shows all aspects of the desired extension. Examine this plan to determine the building materials needed to construct the extension. Write down the different construction materials and quantities, including everything from the concrete to the paint. Call various building supply companies in your area and get an estimate of costs for the supplies, including the cost to deliver the supplies to your building site. Note the average cost of supplies for your calculations.

Step 2

Make a list of the tools you'll need to buy or rent to complete the project yourself. Call an equipment rental shop or home improvement store for rental costs, or just the home improvement store if you're purchasing all of the tools for the project. Note the tool cost for your calculations.

Step 3

Contact the building inspector's office in your area and find out which building permits and inspections you need to complete the project, as well as their costs. Be specific about the extension you're building, as differing extensions have different regulations, which can change the overall cost. Note the total cost for the permits and inspections for your calculations.

Step 4

Include miscellaneous costs in your list, such as the cost of additional land if needed, or the costs of a building loan to finance the extension.

Step 5

Add together the costs for supplies, tools, permit and inspections, and miscellaneous costs. Add an additional 10 percent as a contingency fee to handle unexpected expenses or cost overruns. The total is the calculated costs for a DIY extension building project.


Determine the costs of contracting out the work by contacting several general contractors in your area. Get an estimate of costs from each contractor based on your extension plans.

Things You'll Need

  • Building plans

  • Calculator

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