To locate these plans and index go to Collections > Online Manuscript Index > Files Search and do a search for "sewerage". This will return a list of plans. Choose the map of interest and an index will be available. These plans (not online) show building allotments and houses and include owners' names.
TIP: Go to Collections > Online Manuscript Index > References Search and search for a surname. Any sewerage map reference will be identified along with any other references to that name across the collections.