The reason it is less expensive to finish the attic or basement of your home than to add is that the foundation, exterior walls, structure, sub-flooring and roof are already in place. The cost of the foundation alone, could account for a huge difference in cost.

In many cases you can connect your finished attic or basement space to existing heat/cooling, ventilation, electrical and plumbing systems. It is more likely that your existing systems are positioned and connected in ways that readily include the basement or attic. An addition creates an extra space that must be handled by these systems, especially heating and cooling. It is not unusual to find that an addition requires a booster or a second heating/cooling system.