it depends on so many factors. If you are going to keep existing stairways, kitchens etc. The big question is how much more are single family homes worth compared to doubles. In my neighborhood in Buffalo Ny a single is typically worth 25% more than singles. And usually a converted double into a single will be a larger single than a typically home. An important thing when converting is the design. Get a good architect that knows historical homes. You don;t want to remuddle!