That true/false could be used to filter the selection set with a filter by booo mask node. It would also grab bays pointing perpendicular to your building as well though - IE the face of the building looking towards the building across the street and roof planes. Extruding the surfaces may be a better option.
Get your property lines or building perimeter, offset them by your chosen level of tollerance, create a polycurve from them, and extrude them to a height greater than the context. I would go with property lines if you can as they will usually be simpler and therefore faster - you are about to do a lot of tests and simpler shapes will make life better here.
Break the context down into faces and extrude the faces by the maximum distance from the site you are interested in - could be street width times two as one method. Use a geometry.intersects node on the site extrusion and the list of extruded context surfaces. Then use the resulting list of true/false to filter your original list of context surfaces and you will have the correct faces.