I doubt if you’ll be able to add an upstairs room without a special variance from your Building Department.
There are several issues: 1) Its a structural change so you’ll need a Building Permit, 2) the Code requires a minimum ceiling height, 3) minimum floor area required for habitable rooms, 4) window required in habitable rooms and if it’s a sleeping room, an egress window is required.
- Not only is a Building Permit required, but if the value of the structure increases 25% or more, then the entire structure will need to be brought up to code. (Each State has a different threshold for this requirement, so check with your local Building Department.)
The minimum ceiling height for habitable rooms is 7’, but 50% of the floor area can have a sloping ceiling, but none can be lower than 5’. (See ICC R305)
The minimum habitable room size is 70 square feet and the minimum dimension in any direction cannot be less than 7’. (See ICC R304) Remember, those dimensions must meet the room height dimensions noted above.
You’ll need to add a window (or skylight if not a sleeping room.
Btw, a habitable room is defined as a room used for living, sleeping eating or cooking. Bathrooms, closets, halls, storage, or utility spaces are not considered habitable spaces.
Also, your new stairway must meet the current code too, including maximum riser height, minimum tread depth, etc.
Summary: Regardless of the structural adequacy of your floor and foundation system, there are many requirements that will be difficult meet. I’d recommend meeting with your local Building Officials to insure you are eligible.