noun
bottom floor; lowest floor
Refers to the lowest floor of a building, typically the ground floor or basement level.
このビルの最下階は駐車場になっている。
The bottom floor of this building is a parking lot.
エレベーターで最下階まで降りてください。
Please take the elevator down to the lowest floor.
一階 specifically means 'first floor', while 最下階 can refer to any lowest floor, including basements.
Compound of 最 (sai, 'most') + 下 (ka, 'lower') + 階 (kai, 'floor'). The reading さいかかい follows standard on'yomi.