Quick answer: Unity baked lightmaps showing visible seams between modular wall pieces? Lightmap UV1 packing puts adjacent walls in different atlas regions - enable Stitch Seams on the model import.
Modular wall kits look fine in editor, but the lightmap bake produces a hairline seam at every wall joint.
Enable Stitch Seams
On the model import, expand Generate Lightmap UVs and check Stitch Seams. Adjacent shells in the atlas get matched edge texels.
Bump pack margin
Drop Pack Margin to 1 pixel. Smaller margins reduce bleed between unrelated shells but require Stitch Seams to compensate at the geometric edges.
Add a lightmap parameters asset
Increase Backface Tolerance to 0.5. The bake includes contributions from samples near the seam that the default tolerance excludes.
“Lightmap seams are an atlas-packing artifact. The bake doesn't know your walls share an edge unless you tell it.”
For kits, bake one canonical wall and instance it. The seam appears once and can be fixed once rather than across every variant.