Many older Denver homes lack a bathroom on the main floor making everyone have to traipse upstairs to use the facilities.
To add a small bathroom to your ground floor, take an existing closet and make it into a powder room. You can squeeze a powder room into a very small space. Another bathroom addition idea would be to build new walls to carve the space out of a hallway or other room.
To be most cost effective keep the bathroom next to existing plumbing. So try to build directly next to, above, or below the kitchen, the laundry room, or another bathroom.