A: Where are they now?
Sorry for the weak humor.
There several dimensions of "space" to consider when deciding stair placement. To understand the space needed you have to thing in 3D.
How much space will be determined to an extent by the measurement from the finished floor of the main floor to the finished basement floor. However there are a few other considerations also.
One "headache" will be enough headroom. The opening to be cut into the main floor has to be long enough so that the minimum headroom in all parts of the stairway will not be less than 80" measured vertically from a sloped line connecting the tread nosing (the rounded front of the tread) or landing to the lowest part of your ceiling line in the basement.
Some times you can solve space problems by adding a landing and turning the stairs in a l-shape or an U shape, sometimes you're just eating up other space. Another space consideration is the width. Most codes call for a minimum clear width from finished wall to finished wall of 36". If they are only service stairs that will be fine. If the basement is going to be finished or you have to move furniture or appliances up and down, I would recommend 42" minimum. Remember to add to the clear width the thickness of wall framing and the finish material of the walls.
Your riser height will determined by the measurement from the finished floor of the main floor to the finished basement floor. Keep in mind that most codes determine that maximum riser height should be 73?4 inches. I like a rise of around 7" for a comfortable stairway when possible.
Tread depth will determine the overall length of the stairs from the riser of the bottom step to topmost riser. Code says that minimum depth of tread, where you step, is no less than 10". For service steps that may work, I try for a depth between 10 1/2" and 11" is most comfortable.
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