How to estimate a building extension

Updated April 17, 2017

Adding on to a building can be a difficult task. With the necessary building knowledge, such as carpentry, plumbing and electrical skills, you can accomplish it yourself safely and according to local codes. You also have to read the building plans to properly calculate the types and quantity of construction materials you will need. Estimating the cost of a DIY extension on your own home involves determining the costs of building supplies and the costs of required fees, permits and inspections.

Estimate the costs

Study the building plan and determine the quantity of each construction material the plan calls for. Make a list of all these supplies and call the contractors department of your building supply company to get the price for all these materials. Materials can include lumber, concrete, drywall, plumbing, electrical components, nails, screws, drywall mud, paint, insulation, roofing materials, guttering, and the cost of transporting all the supplies. You should also calculate the anticipated electricity use for the project. Add 10 per cent to the total of these amounts as a precaution to get the total cost of materials.

Call your local authority and find out what permits and inspections are needed and the cost involved in each instance. Find out all applicable rules and regulations so the project can be completed legally. Rates and regulations vary from area to area. Determine the costs involved with all necessary permits and inspections for your specific building extension and add all of these figures together.

Add the cost of your building supplies to the cost of the permits and other fees. This is the figure you will need to estimate the cost of your DIY building extension. A free building cost calculator may also be useful to see if you have estimated correctly.


If the work is to be contracted out, get multiple bids from licensed contractors familiar with your area. Adding in the cost of lost productivity for the time you have spent on the project that could have been used to create income is often a wise move and can help determine how much of the project you may wish to contract out.

Things You'll Need

  • Building plans for new extension
  • Phone
  • Pad and paper
  • Calculator
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About the Author

Billy Ray has been writing since 1994. He writes a popular featured column on the sports Web site Bleacher Report and has been licensed in loan origination and real estate. He is an EPA-certified Lead-based-paint renovator. Billy has taken courses in real estate, commercial lending and home renovation in addition to college courses in writing at Southern Oregon State University.