There are two ways to frame in the area underneath a stairway. One way is to build full-size walls adjacent to the stairway that enclose the areas on top of the stairs and beneath the stairs at the same time. The other is to build triangular walls that fit underneath the stair stringers and enclose only the area beneath the stairs. Which is more appropriate for your situation depends on your renovation plans and the design of your stairway.
Things You'll Need
Screws, 4 inches long
Measure the horizontal distance of the wall you plan to build beginning at the point where the stair stringer meets the floor and ending where the stairway meets a vertical wall or where you want the under-stair wall to end.
Measure the vertical distance at the point where the wall ends on the opposite end from where the stringer meets the floor.
Measure the length of the bottom edge of the stair stringer from where it meets the floor to where it meets the opposite end of the planned under-stair wall. You now have three measurements that create a right triangle, and these three measurements represent the exterior dimensions of your under-stair wall.
Cut a 2-by-4-inch board to the length of the measurement you took in Step 1. Make the cut at an angle equal to the angle of the stair stringer to the floor, so that the end of the 2-by-4 fits in under the stair stringer. This 2-by-4 is the base plate of your triangular wall.
Cut a 2-by-4 to the length of the measurement you took in Step 2 minus 1 1/2 inches. Subtracting 1 1/2 inches accommodates the thickness of the 2-by-4 you cut in step 4. Cut the top end of this 2-by-4 to accommodate the angle of the stair stringer. This 2-by-4 is the end stud of your triangular wall.
Attach the 2-by-4 that you cut in Step 4 to the 2-by-4 that you cut in Step 5. Drive two 4-inch-long screws through the first 2-by-4 and into the end of the second 2-by-4 to create an L shape at a right angle.
Lay another 2-by-4 on top of the two 2-by-4s that you screwed together in Step 6. Mark this 2-by-4 with cut lines that will make it into the hypotenuse of the right triangle. Cut this 2-by-4 and attach it to the other two 2-by-4 boards with 4-inch-long screws.
Fill in the rest of the triangular wall with vertical studs that are spaced every 16 inches. Each of these studs needs to be cut individually because each will be a different length.
Install the wall frame underneath the stair stringer. Secure it by screwing the base plate into the floor and the top plate into the underside of the stair stringer.
Repeat Steps 1 through 9 for the other side of the stairs if it also requires a wall.