Team,
When setting a mast for an 8 foot C band dish how much rear clearance do I need if setting in front of a structure?
That all depends on how the structure is laid out on the property, and how that corresponds to YOUR particular satellite receivable arc "look angle". You'll need to measure, and calculate for yourself what that minimum distance will be for sure.
Let's say that a perpendicular line going out from the center of that building line faces directly your "True South", and that's where you want to install your dish. To be able to allow the polar mount to swing the dish from it's furthest East and furthest West and not hit the side of the wall with an 8ft dish, you'd need about a bare minimum of 5 feet clearance OUT from the building. That's just for the dish to clear and not hit the wall, though I'd recommend you give it a bit more distance than that, especially if you use a linear actuator that's longer than 24".