地下 and 屋上: levels and movement through a building
Distinguish basement and rooftop locations, movement directions, and access restrictions.
A location below ground
地下, read ちか, means underground or the basement part of a building. 地下に駐車場があります。 says that there is an underground parking area.
地下二階に降りました。 means going down to the second basement level. Dropping 地下 changes the location.
A location at the top
屋上, read おくじょう, means rooftop. 屋上から町を見ました。 means looking at the town or urban area from the rooftop.
屋上に上がりました。 means going up to the rooftop. 屋上 is the destination, not the origin of a downward movement.
Movement and restrictions
地下から屋上まで階段で上がりました。 means going up the stairs from the basement to the rooftop. Preserve the origin, destination, and means.
今日は屋上に入れません。 says that rooftop access is unavailable today. Do not expand today's restriction into a permanent ban.
Does 地下二階に降りました mean going to the second floor above ground?
Show the answer and why
No. It means going down to the second basement level.