This would be a good place to start.
Changing the setting to Production vs Preview build for lighting quality invalidates this example. 4.13 gets even better results with less light bleed around the bottom edges. Mostly because the floor is uneven and the walls are not touching in those parts.
Comparison here from left to right: Preview, Medium, High, and Production and all done in 4.12 by only changing this value.
If you go a step further and set the walls so that there is no gaps between the floor and use a floor piece that does not have significant portions outside in different lighting conditions you can get better results. This is a just a single floor piece slightly scaled to the width of the room with minimal overhang into the outside lighting and it is using the default lightmap resolution of 64.
These suggestions have been previously mentioned in this thread.
Edit: Looks like some of the other community members beat me to it while I was putting my post together. 