Cover a cleanout plug with drywall or leave an access panel - what's your take?

Drywall over it, and in ten years you'll be cutting a hole in brand new tile or board to get back to it. That's exactly what happened in my house. Previous owner boxed in a cleanout behind the laundry room wall, and when the main line backed up, the plumber charged me extra to cut trough sheet rock and then never patched it right. Looked terrible for two years until I redid it.

I'd leave it accessible. Either build a small access panel with a proper frame and a painted cover, or just cut a clean square hole and fit a trim ring around it. Mine sticks out about half an inch past the finished wall and honestly you stop noticing it. The square nub on the front means you can get a wrench on it if there's ever a clog, which is the whole point of it being there.

The sticker with numbers is probly just a manufacturer marking or a date, nothing to worry about. If the plastic looks intact and not brittle or cracked, leave it alone. If it's chewed up, replacing the plug is cheap while the wall is open. One thing I learned the hard way: take a photo of the spot and note roughly where it sits between studs, and tape a note to the back of the vanity or somewhere it'll stay. Next owner, or you in fifteen years, will thank you.

Before you decide anything, find out wether this is even the only cleanout you've got. Walk the outside of the house and look for a round plastic or brass cap near the foundation, usually within a few feet of where the drain exits. Mine was hidden under a juniper bush the whole time we lived here. If you've got one outside, whatever's behind your wall is probably a secondary one the original plumber threw in for convenience, and it matters a lot less.

In my last house I found a buried fitting just like yours during a kitchen redo. I traced the pipe, confirmed the outside cleanout worked fine, ran a snake down both to be sure, then capped the indoor one with a proper threaded plug and glued a plate over it. Five years on, zero problems, and the plumber who came for an unrelated issue never once asked about it. If it turns out this is your only access point, then yeah, keep it open somehow. But a lot of people would just assume it's critical without checking. Half an hour with a flashlight around the perimeter of the house coud save you from designing your whole wall around something you don't need.

Before you board it up, pop that plug open while the wall's still torn apart. Flashlight down the hole, run a tap upstairs and listen. If you hear water rushing thru, you know exactly what line it's on and whether it actually works as a cleanout. I found one like that behind my mother's kitchen sink years ago, and it turned out the line was half full of grease from the old days when everybody dumped bacon fat down the sink. Cleaned it out with a bucket and a coat hanger hook, took an afternoon. Much easier to sort that stuff out now than during a backup with a finished wall in the way.

I've been on the leave it accessible side of this twice now. First time I just mudded around it and hated looking at it. Second time, in the basement bath, I screwed a small hinged stainless access door over the spot and painted the frame to match the wall. That door is the only reason a root clog last spring cost me twenty minutes with a wrench instead of a plumber visit. Takes up barely more room than the plug itself, and nobody notices it unless I point it out. If your wall framing happens to land awkwardly for a panel, the doors come small enought that you can ussually make it work.

Worth checking wheather there's even a line behind it before you plan your hatch. I pulled one of those square-nubbed plastic plugs out of a 1978 ranch I gutted a few summers back and there was nothing behind it but a short stub of pipe capped off in the crawlspace. Turned out it was a test cap the plumber never bothered to knock out. If yours is the same, you can cut that section out, cap the line properly, and board right over it with a clear conscience. Pop it and shine a light in first.

Seconding the hinged access door idea. I did the same in my half bath but went a step cheaper and just hung the cutout piece of drywall on a couple of cabinet magnets screwed to a wood frame behind it. Painted it, caulked the seam, and honestly it reads as just part of the wall untill you need it. When the drain slowed down two winters ago I popped it off in ten seconds, snaked the line, done. One tip though, don't paint the plug itself shut or you'll be scraping it with a screwdriver when you actually need that square nub.