Ah, Detail Cell Size. I may take a look at that later 
Thanks.
If you made a Copy of your Lights+stuff and used it inside your grass3 level, it will affect all the other levels too after enabling / loading that level. And you would have twice the amount of light sources inside your level? Am I right there?
What you could try is working with Lighting scenarios. These are levels that you can turn on / off and solely use them for light baking. Move all your light setup into a new lighting scenario layer, enable grass + house and bake the light. Now the lighting scenario should contain all the lightbake data you need. You even can use a second lighting scenario to bake the whole light without the grass, so there would be no strange shadows on the floor if the grass isn’t loaded.
I really hope I’ve got your problem right here 