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The Average Cost to Construct a Basement Addition

Derek Dowell

Estimating costs for a basement addition is not an exact science, with many factors contributing to the final number. In the face of so many variables, it is difficult to provide a cut and dried answer that fits every situation.

Building a basement addition is usually cheaper than adding a second floor.

Basement construction expenses are normally discussed in terms of a cost per square foot of floor space, with a range running from $20 per square foot, if it's a renovation project on which you do some of the labor yourself, to $70 per square foot or more for a full addition created from the ground up.


Perhaps contrary to conventional wisdom, a smaller basement may actually incur a higher cost per square foot than a larger one. The determining factor is whether a bathroom is included. Since this is the largest expense, the price tag must be spread over a comparatively smaller number. Plus the quality of the fixtures and features you include in the bathroom have a lot to do with the eventual cost of the basement.


When planning your new basement addition, you should take into consideration that a square or rectangular shape costs less than one with more than four corners, or includes nooks or crannies resulting in six, eight, or even 10 corners. This makes sense when you consider that more complicated designs result in more cutting, measuring and time required by the construction company.


It's already been mentioned that the average cost for a basement addition can only be estimated, due to variations in size, quality and costs in your part of the world. Still, an example is almost always helpful. The cost per square foot for a ranch house with a basement added on would commonly fall in the $45 per square foot area, which is lower than the cost per square foot it probably took to build the house itself, which could easily be double that. For a straightforward cash outlay estimate, it generally takes from $15,000 to $25,000, according to CostOwl.com.


One thing to keep in mind is that labor represents fully one half of the cost of any type of construction, if you hire a contractor. An easy way to slash the final price tag by 50 percent is to do the work yourself, leaving only the cost of materials and tools. Even if you're not qualified to do all the labor, shouldering the burden for even a quarter of the project goes a long way toward saving money. Another sneaky factor in the equation is local construction costs. The truth of the matter is that building in a place like Los Angeles, California, is more expensive than West Plains, Missouri.