“How much would it cost to build a bathroom (shower+toilet+sink) and a dry wall to separate it from the rest of the house without adding extra sq ft?”
Purchase and installation of a *cheap* shower, toilet and sink is about $1500 and drywall and electrical around it another $1500. Not expensive at all.
The bigger issues are getting the water and sewer pipes there without running them through living spaces, and getting the city permits for everything.
Maybe you should start with some less ambitious projects like repainting the outside of the house, replacing gutters and installing a rain barrel, and making the landscaping beautiful.
