I'm not really worried about the slope, as I not as much concerned about the actual "eye level" as such , but I do want to maintain a consistent viewpoint height, so that the perceived angle of the house is always the same. Again, probably not the best explanation ...
I haven't found the exact place to be able to adjust the setting, but there must be some sort of hard coded setting on that. The reason I suggest this is because if you go into 3D view and place the view angle close to eye level, then begin to zoom in or out, the view angle adjusts automatically to a preset height of some sort and will maintain that unless you manually adjust the camera elevation...