Dear James: We plan to add a half-bathroom, but our house is built on a slab. Is it possible to still add a bathroom there, because there is no drain to the sewer in that spot? —Charlotte G. Dear Charlotte: It definitely is an easier project to add a bathroom in a room over a basement or crawl space, but it can be done over a slab. Running the flexible water supply lines isn’t difficult, but adding the drain to the sewer is more involved. This is the same issue when adding one to a basement. Making detailed plumbing plans initially is key to a successful project. Purchase the vanity, sink, and toilet, and locate them in the room in their desired locations. Precisely mark the location of all the drain plumbing connections on the floor, and then remove them so you have room to work. This project can get …