Hi, i am a beginer but things work not that bad. I have imported an architecture project from blender to get the 2 uv channels, but when i build lights i get a message:
Lightmap UV are overlapping by x%. Please adjust content Enable Error Coloring to visualize
My shades inside the building are really weird but when i check on the forums, that seems i do things correctly.
U can have a look on my print screens and maybe u will fint out how to help me! that would be awesome. Thank you
There are some uv’s in the top left corner that are overlapping/unwrapped wrong, if you fix that, most issues should be gone.
you cannot have uv’s for your lightmap outside of that square, and you cannot have them overlapping or mirrored.
Keep that in mind 
Hey there liviou69
Luos is correct from what I see. Even when painting UV’s the same color, you should never overlap them. This causes an issue where UE4 can’t correctly apply shadows and lighting to the object in those areas resulting in errors when building the lighting. If you need any more help or have further questions, feel free to ask. I hope this solves your issue and good luck!
- Ryan
Yea, the overlapping as not so much the issue here. The real problem is that that object is way too big to be lit correctly, as all the UVs are on a single object, the individual polygons become way too small for the light mass texels.
What you need to do is break it up into a lot of smaller objects so that each element has more UV space to be written to. Just break things up according to some logic; Example, having all the East walls as a single object with reasonably scaled UV’s, so on and so-forth.
Here is an EXCELLENT tutorial (by Rag3Dviz) on a good lightmass/uv workflow for UE4:
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