Grammatically you're right, it can mean that as well, but because there is only one ground-level, it wouldn't make any sense to say "This ground-level". At least I cannot imagine any situation to say it so.
No, we do not use it this way. Starting: "nulláról" (from zero) or "az alapoktól" (from the foundations) but not "a földszintről". Literally, we can begin the cleaning of a house from the ground floor = a földszintről / földszinttől.