What are the conditions for permitting construction into an alley's airspace or into the property of a specific individual?

Primary text

If permission is granted for construction into the airspace of the alley, it is only valid provided that the extent of the projection, both horizontally and vertically, is clearly known (ma'lum al-miqdār). The same ruling applies if the projection is made into the property of a specific, identified individual: it is not permitted without their explicit permission, but it becomes permissible with their permission, whether for compensation or without, provided the extent of the projection is clearly known.