Strange indirect lighting on building

I imported the model of a building from Blender into a fresh level in UE4. Each floor is a separate object but they all share the same material.

When I build lighting, I get the result on the picture below (lighting-only view): the outside walls of each floor have a slighly different --but uniform-- shade of grey.

I know that this effect is caused by the indirect lighting contribution because if I remove the ground plane, all floors have the same color. I am using default lightmass settings and a high quality lighting build.

How can I fiw this and have a gradual indirect lighting contribution to all floors instead of this layered cake effect?

Do you mean lightmap density? See below.

Lightmaps are the ones automatically generated by UE. Each floor is set to a resolution of 800.

Thanks a lot for the link Deathrey! Now I understand what is going on.

I rebuilt lighting with Indirect Lighting Quality = 2 and Indirect Lighting Smoothness = 0.6 and the walls are now nice and smooth.

Can we have a look at the lightmaps? <3

This is known limitation of light baking. You can read more about it here.

Shortly, you have several options: either cover the seam between floors with some sort of trim mesh, or ensure that the seam between modular floors goes with some sort of an angle, or lastly, crank up lightmass quality settings until the issue is minimized to the point, where it feels acceptable.