Size of Addition

• The size of your anticipated home addition space will greatly affect the overall cost. For example, for an entire second floor addition that is 1,500 square-feet, expect to pay upwards of $450,000 if hiring a contractor and $225,000 if doing a majority of the work yourself, as stated on Cost Helper. For a smaller home addition project, such as adding a family room to an existing space, expect to spend $2,000 to $5,000, depending on the square footage of the room, materials needed and any labor costs. Before hiring someone to build your addition, obtain quotes from several contractors based on the square footage of the project. These quotes will give you an idea of what the overall addition project costs.
  
  

Location of Addition

• The location of your home addition is another major determining factor of cost. If you are transforming part of your backyard into a new master bedroom with an attached bathroom, be prepared to face landscaping, construction, electrical and plumbing concerns. If building an entire second floor or bonus room over your garage, ensure that your existing home can support the new floor. This obligates you to maintain proper structural support throughout the project, which requires hiring a contractor if you have no experience.
  
  

Using Existing Space

• Building a home addition in a preexisting space saves you hundreds to thousands of dollars, since the structure is already there and the addition may only require minor work to become a new space, such as pushing walls back and adding more flooring. For example, adding a small, do-it-yourself bathroom to an existing space in your home costs $3,000 to $6,000. To hire a contractor to build a bathroom in a nonpreexisting space, where you need to perform major construction, expect to spend upwards of $50,000, according to Cost Helper.
  
  

City Requirements

• Your city of residence has requirements for construction and home additions, such as a maximum home addition size, hours allowed for construction or any building permits and associated fees. Provide blueprints and project plans to your city to have your home addition cleared before starting spending money to hire a contractor, rent machinery or purchase building materials.