・A set of rooms (including a kitchin and bathroom) within a larger building is 

　called a flat in British English or an apartment in American English.
 
　アメリカでは、アパートメント、イギリスでは フラット だそうな。

更に調べると、

・A cottage is a smalle, old house, especially in the country.

・A bungalow is a fairly modern house built on only one level.

・A small one-room flat is a bedsitter in British English.

・A large, grand house is called a mansion or (if it belongs to a king
　or a queen) a palace.

・Bedsitter は、居間兼寝室一部屋だけの小さなアパートで、イギリス英語らしい。


　コテージ、バンガローとの違い。 アパート、フラットとマンションの違いも分かった。

　アメリカ人に問い合わせてみたら次のような返事をくれた。

・In the U.S., we do not say "apartment house".  The individual

　apartment is called an "apartment".  The whole building (of many

　apartments) is called an "apartment building".

 
　The more I think about it, I think that "Flat" is a European (British?)

　term.  It has basically the same meaning as an apartment.