FLAT is an old-fashioned apartment, usually two floors.

APT is a new apartment building.

The unit is a single house, and several units are on the same ground.

In terms of rent, APT is the most expensive if the number of rooms is the same, the house is old and the location is the same, because the landlord has to pay the property management fee.

Flat, flat, and the rent of the house is similar.