A duplex house plan has two living units attached to each other, either next to each other as townhouses, condominiums or above each other like apartments.

What duplex apartment means?

: an apartment having rooms on two floors.

What exactly is a duplex?

A duplex is a multi-family home that has two units in the same building. These two units always share a common wall, but the floor plan can vary. Units can be arranged either side by side or stacked on top of one another, each occupying an entire floor or two of the building.