When I was living in a flat, I was lucky enough to have a balcony, so I would carry the bike through the building and store it on the balcony, covered with a waterproof sheet. It was a bit of a pain to get it through the building, particularly when the tyres were muddy as I didn't want to leave a mark anywhere, however I enjoy cycling that much that not doing it was never an option for me. If you really struggle for space, you may wish to consider a foldable cycle, as this means that you would be able to put it somewhere else without it having to take up all of the room that you have available to you.