One approach is a steeper staircase, but this is likely a false economy due to the increased risk of accidents. Another would be a fold-out staircase that would not consume space when not in use. A fold-out staircase would typically not be an attractive solution however due to the hassle of folding it out each time for use. However, if such a fold-out staircase could be mechanised, this "hassle-factor" could be significantly reduced.
A simple design would be a set of boards that when not in use are folded upwards against a wall. The banisters would be folded against the boards, and would fold up when the board is in down position. Around the hole in the floor in the upper room an automated guard rail would be required.
A similar approach could be taken with an elevator: the floor of the elevator would default to being in the upper floor; guard rails would be folded away when not required.
Such solutions would require relatively sophisticated technology: mechanisms to efficiently fold away the banisters, rails, etc; sufficiently strong materials; sensors to ensure people, pets and other objects aren't in the way when deploying.
Obviously, the space saved could not actually be used to place furniture or any objects, but would enhance the visual space of the house.