Cost estimation for the rehab

[Jeff Cliff]

Hello,

I am working with a designer/planner to change the floor plan of the house I am rehabbing. I do not have a general contractor yet so I am not quite sure how much to expect as far as the remodel work goes as there are some significant changes I am making, adding sq. ft moving kitchen, bathrooms, exterior walls, removing interior walls, new roof plan etc. in addition to the usual new texture, flooring, paint job etc.

Unless the preliminary plan is done, there is no way general contractors can bid on it but at the same time this being my first rodeo I have no clue how much to expect in this circumstances. How can I figure out what is/isn't within my budget? Should I get cost estimation before going to the city with the preliminary drawings? 

Thanks in advance for your response.