Don’t feel bad about it. Everyone has this doubt. And, to be honest, it’s because the answer to it is very frail.
Here’s the deal. Both “apartment” and “flat” are words used to refer to livable self-contained residential spaces within a larger dwelling. To be more specific: residential property units; with a front door, a kitchen, a bathroom with a toilet and vacant space for you to live (regardless of the number of rooms).
However, language is a tricky thing, right? It’s a living organism that’s constantly evolving. With time, the usage of both words started to take on different or additional meanings. For instance, in North America apartments is usually the go-to word to describe residential units within a building – however, if they are owned instead of rented by the person, they are referred to as condominiums or “condos” and, if it’s a two-story home (or even a townhouse) which was converted into two separate units with exclusive front doors, each floor is called a flat. In the UK, flat is used for apartments that are on a single level – that is; everything that is not a duplex or triplex – hence the word “flat”, while apartment is used for luxury units that have more than one floor.
So, the truth is that it will require context for you to understand when there is a difference between apartment and flat. Because they are like non-identical twins, you know? Although at their birth certificate they are synonyms, most people have built uniqueness to their meaning and view them as two similar but not identical things.
If someone is talking about a flat within a conversation, pay attention to the description of the place to see if the person is talking about a regular apartment unit or if it’s a specific one-floor unit, or even a repurposed townhome. Or just go ahead and say: you know, I’ve learned with the folks at RealEstateAgent.com that apartments and flats are the same thing but people use it in several different ways; may I ask you exactly what is *your* definition of the word, my good sir?
The easiest way to not get confused with the difference between an apartment and a flat is by having a real estate agent by your side so whenever there’s a “flat vs. apartment” situation, you just turn his/her way and say “Translation, please”.