When my lighting is built some of the walls are darker than others.
Here are some pictures.
Could you show your lightmass settings? Do you use CPU or GPU? If CPU, did you change your Lighting Quality Level?
And your Lightmass Settings? (you can find them under World Settings)
Back then I used to have Static Lighting Level Scale on 0,1 and the Indirect Lighting Quality on 10. (Rule of thumb somehow was: if you multiply both you want to get 1,0 as result). Indirect bounces ~ 10. (and disable Compress Lightmaps because of possible artifacts).
Do you have a Lightmass Importance Volume? You could even try to add some Lightmass Portals, covering your windows.
I have changed the indirect bounces to 10 and added a lightmass importance volume and added lightmass portals to the windows and unchecked compress lightmaps. Just wating for it to build now.
Ok. Meanwhile: I used these settings most of the time.
Took me some time now, but I finally found my go-to source again:
So, for a quick read:
Tom Looman wrote a summary about a really nice and official video by Epic about Lighting.
Unreal Engine 4 Lighting Masterclass - Tom Looman
And if you have some time to spare:
Lighting with Unreal Engine Masterclass | Unreal Dev Day Montreal 2017
It is fixed when i build it, it is no longer dark in places but walls have artifacts. Is it the Indirect Lighting Quality?
Could be - try to follow the rule with: Both values (Static Lighting Level Scale and Indirect Lighting Quality) multiplied should give you a value of 1. As in my example, the 0,1 and 10.
Even the Lightmass Compress enabled could result in some JPEG-ish artifacts.
The guide above may help you there 
Ok ill set Indirect Lighting quality to 10 instead of 2.
But thanks for helping me to solve this.
