Measure the square footage of the addition that is being added on to your current home. This is only to get an idea of how large a project you are tackling. For instance, a sunroom might only be 200 square feet while a garage could be as much as 5,000 square feet or more. To find this measurement, measure one side and multiply it by the other side to find the number of square feet. Break down irregular areas into squares and measure them individually and then add in their total square footage.
Multiply the number of square feet by the average cost of construction for your area. If you live in an area of the country where the housing prices are low, you can figure about $80 per square foot while more expensive areas can figure on $200 per square foot, according to the B4UBUILD website. This is the base cost for the addition.
Write down a list of items that will incur extra costs, such as excavations, special windows, in-floor heating, granite counters, new landscaping, permits and specialty items such as alarms and plumbing. The base cost does not include any of these, so be sure to think through the details of your project. Get as many estimates as possible for each phase from professionals to build a real cost estimate.
Add 25 percent for the unseen details that will add up to be quite an expense. If you end up not needing it, then you don't have to spend it, but if you are short, construction might be delayed.