Measure the basement. Start at the exterior walls and measure and mark all of the walls onto the graph using a scale of one-quarter inch equals one foot. Note the location of any windows, walk out stairs (to the exterior of the house), doors or other condition (such as vents) that penetrate the basement walls.
Measure and mark the inside of the basement noting the position of all foundation piers, posts, walls, doors or other obstructions. Be sure to note the location of utilities like furnaces, water heaters, a utility room, old boilers and with very old homes old coal chutes. Note the interior staircase that leads to the main floors of the house if the basement currently has one.
Measure the ceiling height of the basement from the highest point on the floor to the lowest point on any pipe, duct or vent. Then note the average height from the basement floor to the bottom of the floor joist above. Note on the side of the graph if the floor joists (overhead) are insulated.
Locate and mark all of the wiring, plumbing, heating and air conditioning ducting. As you move about the basement make notes on the graph of any problems with the basement like exterior wall cracks, moisture, mold, mildew, standing water or other similar problems.
Read through the building codes that apply for your home, neighborhood, city or county as they pertain to designing and building rooms in the basement of the home. General basement construction looks at ceiling height minimums, escape access through windows and use of the room. If the small room is an updated laundry room or perhaps a room used for a hobby, then having an escape window may not be critical (although it is always a good idea). All bedrooms require egress windows.
Lay tracing paper over the footprint (graph) of the existing basement. Locate where the new room can be placed within that footprint that will meet local codes and your needs. Draft the shape of that room showing how the room is accessed (it may need a hallway to the staircase) and reflecting any other needs the new room may have. If, as an example, the room is a laundry room then plumbing and electrical have to be located relative to the room. The finished small basement room should be finished from the staircase all the way into the room. An exit door to unfinished portions of the basement should also be located on the new plan. Note all of the measurements.