I expect every dojo owner who owned the building has done a lot of repairs and improvements. My senseis bought a pretty run-down commercial space a few years back -- now it has a new roof, new interior walls to divide the space differently, mats of course, a really beautiful kamiza made from local wood (sensei's a carpenter, among other things), a new entryway, and an ornamental garden where there used to be asphalt. This year we might rip up more asphalt to extend the garden -- I'm suggesting we proceed with caution, it's a huge project and we need to make sure we've got the bodies to get it done.
If you've got the right site, skills, money and time, you can build anything you can imagine. Otherwise, what you build will be at least somewhat constrained by the site, the existing structure, building codes, etc.
Picture of the roof-raising
Our first garden planting
(there's a lot more there now)