my new WM only opens from right to left, forcing me to place it further from the old drain pipe.

I've read everywhere that current code requires a 2" drain pipe.

from out of the wall, I see 1.5" pipe.

since it is covered wity drywall, i can't see much more.

My father says the code should be for the main drain pipe and that 1.5" is okay with the vertical riser and the horizontal pipe leading into the wall drain.

is he correct or do i need to break the wall and change it to a 2" pipe?

what if the pipe inside is also 1.5, do i need to replace the whole drain assembly?

is it possible for me to use a 2" pipe and use an adapter to connect to the 1.5 in the wall, in case the inspector insists on a 2" pipe?

house was built in 1965.

he also mentioned that as long as the vertical stack was above the WM tub, there should be enough pressure to drain properly with a 1.5" pipe.

thanks.

first time homeowner here.