Light showing under shape

All I’ve added to my project is a hollow rectangle with a door entrance. Why does the outside lighting light up the bottom sides of the shape?

you need to increase the lightmap resolution of the ground, if the ground is BSP select the surface and lower the lightmap value (which increases the resolution)

When the lightmap is too low, the pixels become very large, so it’s bleeding inside the building. Also, for buildings you need to make sure that they have thickness–surfaces on the outside and surfaces on the inside.

Couple things: 1) make sure you have lighting built. Dynamic preview lighting can a little bit sloppy.
2) Lightmass is a great global illumination solution, but it’s infamous for having leaks like this. For best results, you’ll need to have thicker walls than a single plane. On top of that, you can improve contact shadows by decreasing your Indirect Lighting Smoothness, which typically requires that you increase your Indirect Lighting Quality a little bit. If that doesn’t fix it, maybe consider increasing the lightmap resolution of your floor.

But the thickness of the wall is the issue here.

I’m confused. What is a BSP? BSP as opposed to what?

BSP Brush to make Boxes and primitives like cylinders and such.