Legality of portions

I am planning to buy an upper portion of a residential house in karachi. The problem is some people say they are illegal and maybe demolish. Some say only portions upto 2nd floor is legal. When i told them some older portions are already sub leased they say those leases are dc lease and have no value/unsafe. I need details regarding this. How should one buy a portion and what to look for?

Why not to go for some sort of a flat/apartment. You can get something that you are looking for and completely safe. I can see portion wise ownership of same can involve too much headache.

Thank you for you input. Actually it is unlikely that we can get a seperate/personal roof space with appartments/flats, that is why portion is a consideration