backwater valve

am i allowed to place a valve behind a grey water drain for upstairsdrain
adding new bathroom downstairs . (split entrance, sewage drain on basement floor)
putting false floor to run plumbing would like to be sure??

I have no idea what you are asking about. But normally, the backwater valve would be for the LOWER level fixtures and the upper level would connect downstream from it so they would work normally even if the backwater valve was closed because of a flood condition. An entire bathroom is NOT a "gray water drain" system.

down spout from mainfloor bathroom is in front BWV,
but i think you answered my question

Thanks