I have back to back bathrooms. The current lavatories drain to a wye then to the vent further down the wall (behind the toilets). I want to raise the P-trap for one of the lavatories (putting in a vanity with an open middle and trying to hide as much of the plumbing as possible). I presume I cannot connect the higher p-trap into the existing drain pipe as that would seemingly make it an s-trap (which I think I read were not allowed in the US anymore). So it would seem I have to take down the drywall, run a separate drain to the vent for the higher p-trap. That is oh-so much work for about 8 inches of height... any other options? (BTW is there an minimum distance that the bottom of the p-trap has to be below the bottom of the lavatory? I know it has to hold enough water to prevent the gas from coming up, and there needs to be some working space for the coupler...

You said it: you will have to cut drywall and to raise in-wall sanitary "T".

Do this: Dry-set your sink and drain. Test-install trap. Set it as high as the installation allows. Now measure exactly how high you will have the trap discharge end. Then cut the wall and raise sanitary "T" according to that dimension.

Let me know how you did... Back to you... Milo