Thank you for the info.
I set up a simple floor, 4 columns and a roof. The columns had one baking id and the floor+roof another.
(cam=baking cam to rendertarget)
I rendered to UV2 but it always rendered only a part of the object and also included the texture.
How can I calculate the lightmap only ...

Hi,
baking works great if every scene object has its own texture map with UVs ...
but can you please explain how to bake lightmaps for complex scenes and reusable object prefabs using tiled textures?
Is it possible to group similar object into a baking group and thus render one lightmap for several ...