Oh it can still look a lot better! 
You should turn off lightmap compression! /or any ambient occlusion if turned on/
I guess your ceiling is touching/intersecting the walls… try to make it just the visible shape (with thickness) snapped at the corners of the walls!
Also if you’re making your own lightmaps: keep just the visible faces on the map and leave the rest outside of the uv area!
…till you’re doing just testrenders… I would go with 0.1 10 10 2 0.6 lightmass settings (quality is the one that kills your building time but it’s also responsible for a clean result…)
And also no 2048 res lightmaps while you’re just testing… also you can stick with medium build! These should speed up your testing builds but would still get “dirty” but detailed results!