What defines a duplex?

[NM_Shooter]

OK.. here is a weird question. 

What defines a duplex? 

Can a duplex have any shared living spaces at all?  For example, a large living room, perhaps with lockable doors for private entry?

Does a duplex have to have some sort of barrier up through the attic that seperates those spaces as well?

I'm wondering how hard it might be to take an existing home and re-model it to be a duplex. 

[rocking23nf]

2 seperate homes around here, with a connecting wall in the middle.

each pay property taxes, each has own utilities, theres no sharing anything.

[UK4X4]

In the UK which will probably be diferent too

if dividing a property the dividing walls needed to be fire proofed from both sides

and included the attic space- again fire proof.

Usually 2 x 12mm plasterboard each side with fire proof insulation - ground to the roofline-and if it had a shared hallway then each "apartment" that shared off the area had to have a fire door too

But if you simply wanted to rent out half the house "inc bills" then just a standard wall /door would suffice- not required in the attic as it becomes a shared home.