Let me start off by saying, good on you for looking up these terms. Because if thought there was a difference between the three you’re absolutely correct. Some have small kitchens known as kitchenettes, others got an isolated place for you to put a bed, and for one these tiny apartment variants all your amenities are crammed into a single room. Definitely a smart call to do your research before signing that dotted line. Because often times despite legality issues landlords will try to pass off an apartment as something its not, and that says a lot about what type of service and integrity you can come to expect from them while leasing one of their rentals.
There’s two namesakes for this type apartment: bachelor and efficiency. Which one you hear isn’t so much regional as it is a generational gap. Older folks tend to call them a bachelor’s apartment, but most of the time they’re referred to as efficiencies. By definition these apartment combine: kitchenette, living area, and sleeping quarters into a single room. The only thing that distinguishes and efficiency from a room in a boarding house is having a private bath on suite.
Studios and efficiency apartments are very similar in that legally speaking both are a single room. With the difference being you’re more likely to have a full-sized kitchen and dining area along with a secluded place to act as your bedroom. Most of the time they’re rented to tenants who want to live in a swanky area but cannot afford a 1-bedroom apartment in that neighborhood.
You know, much like the studio and efficiency apartments a loft is legally speaking a single room, and also has all of the amenities you’d typically get with a studio apartment. With the caveat being that the ceiling is vaulter and your sleeping area is lofted above the main living area bunk-bed style. You typically don’t see this type of tiny apartment outside of cities were square-footage comes at a high premium. Because of the locations they’re associated with they are seen as a cut above studio apartments.