,,,,,,,,,,,,,,,,,,Flats exist on the south of Uhuru highway whereas Apartments are to the North

flat sounds cheap, low class.
apartment sounds classy, upper class.

Under the Kenyan real estate market context, an Apartment may have extra features like master bedroom ensuite, fitted kitchen, pantry, visitor's cloakroom, marked parking and so on. A Flat on the other had, could be 2 or 3 bedroom with a shared wc/shower. The extra features makes Apartments costlier that Flats, whether renting or buying. Location too adds up to the total cost.

a flat is one where u can readily hear your neighbour landing, kids have mucus running on the nose and clothes are hung outside the kitchen window on each floor!

There is no difference. It means you have 20 families living in one big 3, 4 or 5 storey building situated on a quarter acre, half or one eighth plot. The tenants/owners share the staircase, gate and parking, and sometimes a pool.