You actually need about a 30-35 degree arc of the sky to obtain signals from all 5 orbital locations and about 15 degrees if you live in an area that can get all channels with the Slimline 3.
So if you take the azimuth setting for your area for the dish type you are looking at using, you will need a minimum of 5-10 degrees to the left with anywhere from 5-10 degrees to the right for the Slimline 3 to 25 degrees for the 5 LNB dish.
I know somebody is going to refer you to dishpointer.com, but that will not tell you if you have enough of an arc open for all the orbital locations.
If you are talking about obstructions to the reflector dish itself, then yes, it needs more than that area to ensure that it can obtain reliable signals.