Hi. I wonder why these light-maps spilling light. 
sorry for bad English.
Could you post your light setup + do you get any errors when you build the light? 
I used default settings with production quality level. no error with light-mapping.
Dynamic or static lightning (I think static, but I just want to go sure ^^)? Is this room one big mesh or parts?
-add a lightmass importance volume
-increase the lightmap resolution of your mesh
It’s safe to assume its static lighting since he is having light mapping issues. 
I just started with Lightmass last night and ran into the same issue, even with lightmap resolution at 2048 and added lightmass importance volume. I guess with Lightmass the building has to be a single mesh to get clean result? I have many parts to my building(separate roof,floor,walls,support beams).