There are a number of details you must be aware of when baking light on models which use the LOD (level of detail) feature.
Baked indirect light on LOD models only works correctly when Realtime Global Illumination is switched off. If Realtime Global Illumination is switched on, the lower LOD models from the LOD group will not be lit correctly.
When you are using Unity’s LOD system in a scene with baked lighting, the highest level-of-detail model out of the LOD group is lit as if it was a regular static model, using lightmaps for the direct and indirect lighting.
For all the lower level-of-detail models in the group, only the direct lighting is baked, and the LOD system relies on light probes to sample indirect lighting.
This means if you want your lower level-of-detail models to look correct with baked light, you must position light probes around them to capture the indirect lighting during the bake.
If you do not use light probes, your lower LOD models will have direct light only, and will look incorrect:
To set up LOD models correctly for baked lighting, mark the LOD objects as Static for lightmapping:
Place light probes around the LOD objects using the light probes component.
After baking the light, your lower level-of-detail models show correctly show the indirect and bounced light, matching the highest level-of-detail model:
You should also be aware that only the highest level-of-detail model will affect the lighting on the surrounding geometry (for example, shadows or bounced light on surrounding buildings). In most cases this should not be a problem since your lower level-of-detail models should closely resemble the highest level-of-detail model.