New layout of existing building

I am renovating a 13,500 ft2 building, and am looking for a flexible layout - I need a plan which will provide for the building as a single use residential building initially, with the option to easily convert into 3 or 4 independent units with a communal ground floor/basement space in the future. The new floorplan should arrange the bedrooms as 3-4 bedroom suites, each with a shared communal space. One of the major design issues is the location of the central staircase. The building is laid out over 4 floors, and the current main staircase does not allow access to the basement at its current location, so this is something we need to work on. As the building is existing, there are some structural elements which need to be worked around. I also have a small window of time to change some window locations (new windows ordered and not yet in place) to suit the plan. I would like to work with someone who has experience of working with existing buildings, is aware and familiar with IBC and IRC.