Part of the problem is the foundation. You either need to pour a large foundation ahead of time (that you won't use for a while) or pour it as you go. It's not an insurmountable problem, but it does make it more difficult. Although if you decided to build up instead, by just adding more floors you could avoid that problem.
Is pouring more foundation next to existing foundation problematic and difficult, or is it just more expense to get the concrete guys back out to do more work?
It can actually be very problematic and difficult, depending how how easy the new slab location is to access now that you have a building in the way and presumably landscaping you want to preserve. This can end up adding a considerable amount of expense to a job compared to a bare site.
It’s certainly worth considering. A 20x20 concrete slab can be poured for a few thousand US dollars by a professional concrete contractor, and you could include radiant heating in the slab. Seems to be at least an straight forward option.
