Old School RuneScape uses the British convention for floor numbering: Ground floor, first floor, etc.
A Premade sgg is a drink that can be bought at Blurberry's Bar in the south-east corner of the first floor of the Grand Tree. Premade drinks cannot be used in the Gnome Restaurant minigame. When drunk, it increases Strength by 4, decreases Attack by 3, and raises a few hitpoints, just like the Short green guy drink that can be made with the Cooking skill.
