The cost of adding a second floor addition to an existing home starts as low as $70 per square foot and ranges up to $300 per square foot when using an upper range contractor, according to Cost Helper. Adding a second floor also results in additional tax and contractor fees.
In general, building a second floor on an existing home costs up to 20 percent more per square foot than the price of increasing the existing ground level living space. In addition to basic construction costs, a second floor requires a sturdier foundation and support, according to Better Homes and Gardens. Other costs include proper framing and weather-proofing of the addition.
For basic do-it-yourself second-story additions, a project typically averages between $70 and $150, but Cost Helper warns that this method takes a much greater amount of time, with higher risk of problems. If hiring a contractor, a typical addition includes two to three bedrooms and one to two bathrooms, and this costs an average of $100 to $300. When a more complicated or detailed addition is necessary, the value increases to an average of $400 to $500 per square foot. Additional variables include the quality of materials, the length of time required and the location of the work.