The smallest self-contained apartments are referred to as studio, efficiency or bachelor apartments in the US, or studio flat in the UK. These usually consist of a large single main room which acts as the living room, dining room and bedroom combined and usually also includes kitchen facilities, with a separate smaller bathroom. A bedsit is a UK variant on single room accommodation which involves bathroom facilities shared with other bedsits. In Korea, the term one room (wonroom) refers to a studio apartment.
Moving up from these are one-bedroom apartments, in which a bedroom is separate from the rest of the apartment, followed by two-bedroom, three-bedroom, etc. apartments (apartments with more than three bedrooms are rare in most rental markets). Small apartments often have only one entrance.
Large apartments often have two entrances, perhaps a door in the front and another in the back, or from an underground or otherwise attached parking structure. Depending on the building design, the entrance doors may be connected directly to the outside or to a common area inside, such as a hallway or a lobby.