A lot depends on whether you are changing the basic structure or just rearranging interior, non-bearing walls. 

If you are expanding the size of the building, the cost per square foot of the additional space is probably equivalent to the cost of a new structure, maybe even somewhat more in order to tie it into the existing building.  but you usually have water, sewer, electric etc available on site to offset those costs.

In any event, I did not intend to suggest that by scheme would be cheaper, just hat it might possible better serve the OP  community.