A Maisonette is a set of rooms for living in, typically on two storeys or floors of a larger building and having a separate entrance of its own.
What Differentiate a Flat Apartment from Maisonettes?
A flat is a set of rooms for living on a single floor only, which are typically accessed via a shared entrance and internal common parts.
Another difference between a flat and a maisonette is that with the flat you have your front door leading out onto a common area shared by other flat owners and you need to travel through the common area to be able to exit the building or vice versa.
On the other hand, as a maisonette owner, your front door is your own and you use it to enter/exit your maisonette directly.