Hi,
The lightmap resolution of the meshes, especially the floor, need to be increased. The default lightmap resolution is 64, but will need to be higher. Try something around 128, 256, or 512.
You can use the Override Lightmap Resolution option in the Details Panel when you select the mesh. This will override it for that particular mesh and not all instances.
For additional information you can see a Wiki troubleshooting guide for lighting that I had put together a while back:
I hope this helps.
Tim