You Installing an Air Admittance Valve?

I live where there's zero residential building inspections.......thank God!

Not a real smart plumber. This is the reason all homes need to be inspected, so they don't cause problems for the people living in them down the road. Empathy to those areas that still don't have any inspections done during building. On this one he can't really put an access panel unless he frames off the valve and air seals around it to the walls. Assuming it is on an outside wall that will get insulation, and needs to be sealed up against moisture, pests and maybe a fire barrier depending on the property lines, putting a access panel would cause problems. He could always direct the pipe through the wall maybe with a rain hood so long as it isn't under a window or vented soffit. The gases coming out of DWV are really bad. Or, maybe hire a real plumber and run the pipe through the home, through the attic and terminate above the roof.

IF I ATTACH THE WASHER MACHINES P TRAP TO THE 3 INCH DRAIN DO I NEED THIS VALVE?