What is the origin of the word 'flat'?

In the United Kingdom, Australia and many other parts of the world, the word ‘flat’ is used to mean an apartment or a house. It is derived from the Scottish word 'flet' which means the interior of a house. In Old English, flet means house or floor. The word flat may also be linked to the "flatness" of the abode.

United Kingdom, Australia, Apartment, House, Scottish word, Interior of a house