Where the owner of the premises and holder of encumbrances thereon agree in writing, properly acknowledged for recording, that they will waive all damages that may result from the taking, destruction or removal of the building for street purposes, the Council may, to avoid hardship or to more closely conform to existing buildings nearby and for other special reasons, permit some encroachment within the building lines by special permit, if the location of the building will not interfere with proper and adequate light, air and visibility of nearby buildings and streets.