big, spacious
modest
cramped, little, poky, small, tiny
comfortable, cosy
beautiful, nice
luxury, posh
shabby
modern
self-contained
purpose-built
furnished, unfurnished
one-room, studio
one-bedroomed, two-room, etc.
bachelor, granny
Even the prices of small bachelor flats are unbelievable.
They converted two rooms of their house into a granny flat for Tony's elderly mother.

attic, basement, bottom, downstairs, first-floor, ground-floor, penthouse, top, top-floor, upstairs
the people who live in the downstairs flat

high-rise
a block of high-rise flats

empty, unoccupied
rented
council
holiday
next
They live in the next flat.

block
The tall blocks of flats dominated the skyline.

have, own
They have a flat in Paris and a house in Normandy.

rent
The musician rented a flat in a fashionable area of London.

let, sublet
The landlady found they had been illegally subletting the flat.

buy, sell
find (sb), look for
Do you think that the council could find me another flat?

get (sb)
We got her a flat in the same block as ours.

live in, occupy, stay in
Our flat is one of the two occupied in the block.

share
move into
move from, move out from, move out of
build
decorate, refurbish
a contract to refurbish 18 council flats

lock, lock up
let sb/yourself into
She let herself into the flat with the spare key.

leave, let sb/yourself out of
break into
evict sb from

be located
The flat is located in a modern development.

face sth, overlook sth
a luxury block of flats overlooking the marina

at a/the flat
I'll meet you back at your flat.

in a/the flat
She lives in the top flat.

convert sth into flats, divide sth into flats, make sth into flats, turn sth into flats
The house has now been converted into flats.

coastal (esp. BrE), tidal
mud (usually mudflat), salt, sand
mud and sand flats rich in animal life

on the flat
These birds live on the tidal flats.