Quick answer: The mesh lacks proper lightmap UVs — bakes get overlapping or stretched samples. Enable Generate Lightmap UVs in the model importer, or unwrap a UV2 channel in the DCC tool.
A newly-imported building bakes with splotchy shadows and color bleeding. Its lightmap UVs overlap or share islands with the diffuse UV channel.
Lightmap UVs Are a Separate Channel
Baked lighting samples from UV2 (the lightmap channel), not UV0 (the diffuse channel). If UV2 doesn’t exist, Unity uses UV0 — which usually has overlapping islands packed for texture reuse, perfect for diffuse, terrible for lighting.
Generate in the Importer
Select the mesh asset → Model tab → Generate Lightmap UVs. Unity unwraps a non-overlapping UV2 automatically. Tune Hard Angle / Pack Margin for finer control if needed.
Or Author in the DCC Tool
For hand-crafted control (large architecture, distinctive shadow areas), unwrap UV2 in Blender/Maya yourself. Pack with extra margin so neighboring islands don’t bleed at low lightmap resolutions.
Lightmap Resolution Per Object
After UVs are correct, scale up Scale In Lightmap on the MeshRenderer for objects that need more detail (hero walls, landmarks). Keep it low on distant or low-importance geometry.
Verifying
Re-bake. Lighting is smooth, with sharp shadow edges where appropriate. No splotches or color bleeding between adjacent UV islands.
“Bakes need a clean UV2. Generate Lightmap UVs at import, or author them deliberately for hero meshes.”
For shipping projects, author UV2 in the DCC tool on important props and let Unity generate for everything else — best of both.