Anti-flow Valve

[AZMike]

Thanks. So behind the toilet?

Yes. If you look at the floor plan in by signature, it's below the linen closet.

[RickLight]

How many number?

There's no real answer to that. Most counties and incorporated cities can enact their own codes. I'm no plumber but I believe it's against the international building code that is the basis for the vast majority of adopted codes nationwide. And it's just a good idea.

Code enforcement is a whole other discussion.