Basically, import the floor plan ( PNG is better ) as a texture. Make a material and just plug the texture directly into the color node on the material. Drop the material on a cube in your level and you can work on top of the cube, as a guide.
You will have to juggle the size of the cube to get a working area you’re happy with, but also put a TextureCoordinate node feeding into the texture in the material, because you will want to tweak the UV scale to get the texture to look right on the cube.