Then you have already taken the first steps. If you do not want to rely on the statement from the office, submit a preliminary building inquiry, otherwise
Obtain a demolition permit. Then commission a publicly appointed surveyor with the subdivision. Carry out the demolition, carry out the subdivision, submit the building application, sell the plots, build the house and move in.
Your planner will take care of the details, whom you of course need to involve for the preliminary building inquiry or the demolition application. I think the project is too complex to cover all aspects in the forum.