Adrian nods on Wren's observation and says; "Indeed, some nobles may had purposefully created passage to basement onto sewers to carry out subtle businesses or planned use them as escape route to sewers in case if city falls to siege. Similar floor plans had been used in various other cities, like Cormyr example. So for noble house which lacks such trait is unusual but not unheard. If they have feeling that sewer entrace may create a unwanted entry for burglars."