If you're buying an existing house, make sure the house is situated on the lot in a way that would allow construction of your shop later possible/economically feasible. Certain things to keep in mind:
slope of the land
septic fields (my house has the primary and has to have a certain amount of land set aside as a secondary, meaning I can't build on that chunk of land)
well
setbacks from property lines
HOA restrictions (my HOA allows detached garages, but if the roll-up door faces the street, it must be 30 feet behind the rear foundation line of the house. If the roll-up door does not face the street, then it must be behind the rear foundation line but the 30 foot distance is eliminated).
utility lines/easements
HOA might require the exterior of the shop to match the house, which if vinyl siding isn't a big deal. If it's brick then it gets more expensive, at least where I am.
If you decide to build the house and the shop, then it might be a little easier to get HOA approval and things can be situated to your liking from the start. You just have to be vigilant because contractors, house building companies, subcontractors, etc don't always communicate well. We built our house about 4 years ago, but not the detached garage. I don't know how many times I had to remind various people within the same home building company that there was a plan to build a detached garage later so we needed that to be considered when the house was sited on the lot, when the propane tank was buried, etc.
