Baltimore County has an
Urban/Rural Demarcation Line that limits extensions of water and sewer service, something like what Minnesota's Met Council seems to do. It helps that Baltimore City owns and runs the huge water and sewer system (including all three main reservoirs) that serves the bulk of the metro area's built-up areas. The outer counties have their own systems of pipes but buy a large portion of their water from Baltimore's system.
The State of Maryland has
Priority Funding Areas, which are areas that get first dibs on any state development money and infrastructure investments, with fewer restrictions than rural lands outside the PFA's. If you want to develop subdivisions outside one of the PFA's (whose boundaries are defined, often rather generously, by the individual counties in their comp. plans), you have to pay out of your own pocket for pretty much everything. At least that was the original intent. I haven't followed it much, so I don't know if the current governor has weakened the program or not.
Wow, I just looked at Alleghany Co's...it's huge. Not that more than a fraction of that mountainous area will ever be developed; it's a depressed county and kinda desperate for some growth. They must have expanded it recently, I don't remember it being so big.