This is a very difficult question to answer. Many variables such as the build site, the finishes required, whether or not you consider landscape, design cost and taxes to be part of the cost per square foot. One should be wary of someone giving a blanket square foot cost without substantial information of your specific project. We recommend having a preliminary meeting to get an understanding of the clients expectations as well as their expectations of us.
Generally no unless there is a substantial difference in the size of the windows and structural changes are required.
Fixed would require every last detail of the house before hand and costed out by the contractor. This would result in change orders for any change in material or design or anything that would affect the contractors initial price resulting in extras.
Cost plus – this is strictly whatever the building costs plus a percentage. Which is how the contractor makes his money. Not necessarily in his best interest to keep your costs low, as the higher the cost the more money he makes.
Management and cost – cost is flow through billing. Whatever the contractor or project manager is invoiced by the subtrades the client sees and pays only that price. The advantage is the client receives all the discounts that the contractor has with subtrades and suppliers. This means the client can make changes without fear of being unduly charged for extras or a percentage being added to their change. The manager in this case makes his money based on a fixed fee determined before the project starts. This only changes if there is a substantial change in the scope of the project.