Using GIS for Interactive Floor Plan

Do you have your "backdrop (raster)"?

If yes, then there is two options:

1. it is georeferenced (in some non-earth coordinate system). Then everything is ok and you have just to use it with your vector data (if you have it already) or draw vector "polygons that will represent the rooms" using your raster as a background layer. In first case may be you will need to do additional georeferencing to match raster and vector layers. Be carefull in choosing the right data for georeferencing - raster or vector - you should leave accurate data intact.
2. it has no georeferencing info. Again there are two options:
   • you have vector data ("polygons that will represent the rooms"). You need to georeference your raster data using vector as control points (i.e. room corners)
   • you have no vector data. You need to calculate 3 points from your plan manually using some additional info about your rooms (rooms size), draw that points as a vector layer and then use them to georeference your raster.

Additional info for the last case (no georeferencing info, no vector data):

Actually I am not an everyday user of QGIS but I have found one tutorial which is very close to your task.

In your case you will be creating a point shapefile without any background raster (skipping step 1).

Draw first point in any place. Then take a look over your plan. Lets say that you have a room with size 4 x 5 meters. Place your second point shifting it in 4 meters by X coordinate. Then repeat it for the third point - shifting it from first point in 5 meters by Y coordinate.

For more accuracy you can place your point manually entering coordinates. In this case their coordinates will look like: Point1(0, 0); Point2(4, 0); Point3(0, 5).

Then you should georeference raster using appropriate room corners from it and previouslly created vector points.

Have you considered using Google Map Floor Plans?

It might save you a lot of headache in the long run.