I want to know how this process works, how much time takes and  how much costs. 

It would be great to get connected to people that can handle it,and did in the past i guess its lawyers job. 

Hi Alex!  Sounds like an interesting project.  You could do a lot of initial work yourself like getting permits, having an architect put plans together and talking to a good local contractor.  You would want to bring in a real estate attorney so that they could review everything and sign off on it for you. The time and cost really depends on the size and scope of the project.  

Why would you do that? I'm just curious. Whenever I see a duplex or triples converted to a SFH the first thing that comes to my mind is always "somebody just destroyed a perfectly good investment!"

the whole neighborhood is currently beign rehabed. there is small amount of duplexes in the 0.5 mile area and most of them converted to sfh , repaired on high level and sold for good amount of money