Old-Melbourne duplexes that share a roof - how do you go about renovating?

Interested in a ~100 year old property in Melbourne that shares a roofline with its neighbour.

I'm keen to know whether there is reno potential (to add a second story, or to gut some of the inside and bring it into the 21st century with double glazing, proper insulation etc.

As it's a duplex (is this the right term?), I'm assuming you can't really do structural renos without impacting the neighbours; any advice on how this works? Anyone have experience with this?

The roofline looks like this:

 

A good thing to check on first is whether the dividing wall between the properties is single or double brick. If it is single brick you have a major problem trying to sort out supporting an attic room/second storey and council may have concerns about fire resistance. A single brick wall is not acceptable in a new build but was used in houses of the age you are talking about. Similarly modern standards require a fire rated barrier in the roof space between houses and this was not required in the distant past.

 

I’ve seen a place like this in Prahran, Melbourne - where they demolished one side completely to the dividing brick wall.