Lighting Patches

Hey! The usual culprit for these kind of artifacts are invalid lightmap UVs and/or invalid lightmap texels. Did you author your lightmap UVs yourself, or did you let Unity generate automatic UVs for you?

Try inspecting your scene using Texel Validity scene view mode (accessible through the upper left corner dropdown menu in the scene viewport). If after rebaking your lighting the visualization shows a lot of red texels, then you have texel validity problem. That issue can be mitigated by tracking down one-sided geometry, and enabling Double Sided GI in its material settings.

 

The texel validity marks valid areas as green, invalid as red and undetermined areas as orange. In your screenshot you're not seeing any results at all, so there's not way of telling what the actual state of the scene is. You should also lower the lightmap resolution to something like 12 while you're trying things out, 60 is much too high for experimenting with the scene, assuming you're also using a unit of 1.0 as 1 meter for scale.

 

When we shoot a ray starting from a surface point, and we hit another surface, we check the normal to make sure that we've hit the surface from the outside. If the normal points in the wrong direction, we check whether the object is double sided (essentially flipping the normal). If that's also not the case, we determine we've hit an invalid surface. This usually happens if you've setup your scene with intersecting meshes, e.g. you have a cube halfway in the ground. The surface texels of the ground that are covered by the cube will now hit the cube from the inside, so those will be flagged as invalid. The ground outside the cube will hit the outer side of the cube, so they will be marked as valid. In your case, if all the normals of the walls actually point outside, and the walls are not double sided, the lightmapper will keep hitting the wall from the backside (as that's what the normal is telling it), so it will mark the hit as invalid.

Setting your material to double sided can be a quick way to determine if that's the problem, but actually fixing it would be flipping the normals on all broken surfaces.

The backface Tolerance parameter is a measurement which you can use to allow e.g. 20% invalid ray hits, before the texel itself gets marked as invalid. For your specific setup right now, you can ignore that value.

 

You are probably misinterpreting the invalid texel view. The red texels on the ground plane mean that the rays shot from there hit a lot of backfaces. So there's some other object in that scene that is causing this. Setting the ground plane to double sided won't fix this, you have to find that other object visible from the ground plane that is causing the issue and set that one to double sided (or fix the surface normals on the mesh).