While (currently) factory buildings aren't required to be built on foundation (which makes sense, given that they include a floor), buildings can't be built directly on the factory building floor, requiring building a floor on top of the floor.

For the least hassle for you, I suggest you make a per-surface exception for the surface used by the notnotmelon fork of Factorissimo, as all factory buildings exist on it at once. (That method should also make BRF work properly with mods like Warptorio, where each non-planetary surface has a floor.)

Does that make sense to you?

Hi Keith,

The reason why the Factorissimo building does not require foundation is that under the hood it is actually a storage tank, which are excluded by Buildings Require Foundation.

They way this mod works is by adding collision masks to tile and entity prototypes, which can't be changed after the game loads and thus it is not possible to implement a per-surface exception. I would have to check if it is possible to implement this for Factorissimo and Warptorio.

I tried to implement something in the latest version, can you try it out and let me know what you think?

It still says 'Cannot build on factory floor'.