IMHO there should be extra incentives for BIPV just to the amount that would offset the classic shingles underneath because not just doing a simple barn roof (single gable, two pitch) or triangle roof (single pitch) with the slope entirely covered by solar glass of usual 400~800 Wp size modules is where a big part of the excess wasted cost is from. Just make sure to allow structures that allow module-sized parts of the grid to be replaced with human occupancy windows in a nice and simple way.