A three-foot stairway will take out approximately 50 square feet on each of the floors, which is a 100 square feet. In addition to the cost there is the substantial inconvenience of having the stairway. A stairway almost always dictates a certain amount of house design. It is difficult to place in many houses without interfering on one level or the other. Very often I am asked about basements.
Basements are a hold-over from earlier time periods.
For conventional homes they are useful as storm shelters. A basement is usually a dank, dark hole we bury junk in. Often we have mom carrying her laundry up and down the stairway, so she can wash in the basement. It has been my experience that the money spent on a basement could be far better spent on the main floor, making a larger house. You might gather from some of my descriptions that I am against basements and upper floors.