Stairs should be calculated both upstairs and down. For example, a new 1st floor extension would require the ground floor around the stairs to be renovated. So, downstairs renovation area would include the stair area and upstairs extension area nominated would also include the stairs.
Similarly for a ground and 1st floor extension, the ground floor extension area includes the stairs area while the 1st floor extension also includes the stair area. That is, the stair area is counted once for ground floor and once for 1st floor. This also applies for new builds.