The cover plate is likely an access for unclogging the soil stack and main drain. With the right combination of parts, you could probably configure a laundry drain here, but you must include a p-trap to block sewer gasses and you would not want to defeat the availability of that access for drain clean-out.

Thanks for the info Randy. It sounds doable.

I'm almost positive I've found the location of the old laundry. I'm looking for the previous washing machine drain setup. What am I probably looking for? I know it's got to be there, or some evidence from it's previous location. I don't know what I'm looking for. If there was a tub, there should be something left as evidence. Or if plumbed directly, something there? Much of this is behind a dry bar they built in, so I don't have great visibility, especially not knowing what I'm looking for. And ideas what I should be finding as evidence of the old drain setup?
Thanks.

Code prohibits connection a fixture arm or waste arm to a vertical stack below a watercloset (toilet) served by that stack.

If you connect to that stack code will require a separate vent between the laundry standpipe trap and the stack.

That vent must run up through the roof or to a level at least 6" higher than the flood level rim of the highest fixture served by the stack before tying into the stack.

"What am I probably looking for?"

Basically you're looking for a drain that looks "tall" with a trap, or, for that matter, any drain plumbing in the wall (if there was ever a sink there). If a finished wall was built later, you might open the wall and get lucky, finding something in the wall....but maybe there's nothing there to begin with.

Lazypup, if one were to connect (per code) to that stack and vent separately, where would the tie-in be made? Does one basically have to bust the floor and do it under there, or can the connection ever be made above the floor?

In order to properly answer your question we need to begin with some basic code definitions.

TRAP-by code definition the trap is the U shaped section of the fitting that retains water to affect an air & gas tight seal between the fixture and the sewer line to prevent sewer gas from entering the structure.

TRAP WEIR- just as the weir of a dam is the point on the spillway where water can overflow the dam, the trap weir is the physical point on the discharge side of the trap U section where water passing through the trap can spill out of the trap and into the trap tailpiece & drain line.

FIXTURE ARM or WASTE ARM- While most people think of a fixture arm as the short section of pipe extending out of the wall where a sink trap is connected, that is not correct. The fixture arm is the total developed length of pipe from the trap weir to the vertical stack or vented horizontal drain line. Understanding that it is only one line, it then stands that the line must serve as both the drain & vent for the trap.

Under the International Residential Code (IRC) the bottom of the fixture arm at the trap weir may not be higher than the top of the pipe at the vent opening, thus we can mathematically compute the maximum length of a fixture arm by dividing the diameter of the pipe by the pitch of the run. Example- The code standard for a bathtub drain line is 1-1/2" and all lines 3" or less are required to have a minimum pitch of " per foot thus the maximum length of the tub fixture arm would be 1.5" / .25Â = 6Â but be very careful here. Keep in mind that the top of the pipe at the vented drain cannot be lower than the bottom of the pipe at the trap weir so if you mistakenly run the line with a greater pitch, it will shorten the allowable length of the line and under no circumstances may you have a vertical offset. In addition, the code says, "Maximum developed length of pipe". (I have seen instances where you get a real hardnosed inspector and he/she will make you also compute the fitting losses. By example a 1-1/2" 1/8bend has a fitting insertion loss of 6" which means that while the 1/8th bend is physically only about 3" long, you have to compute it as 6").

The Uniform Plumbing Code (UPC) began with basically the same mathematical maximum as the IRC but then they de-rated the maximum lengths by about 40% thus we must either memorize the lengths or keep the code table at hand.

UPC Table T10-1
1/14"ÂÂ2Â6"
1-1/2"Â..3Â6"
2"ÂÂÂ.5Â
3"ÂÂÂ..6Â
4" or larger 10Â

The minimum length of a fixture arm when measured from the trap weir to the vertical riser, stack or vented horizontal drain is to be 2x the diameter of the fixture arm pipe.

Now to answer your question; the tie in can be made at any convenient point on a stack or horizontal line, except, an un-vented fixture arm MAY NOT be connected to a vertical combined waste & vent stack if there is a watercloset (toilet) discharging into that stack at any point above.

The solution would then be to install a separate vent on the line between the trap and the combined waste & vent stack. By doing so the line from the trap to the vent is classified as a fixture arm, but the line downstream from the auxiliary vent to the combined waste & vent stack is now classified as a vented branch and that may be connected to the stack but be careful here as well. The auxiliary vent must rise to an elevation at least 6" higher than the flood level rim of the highest fixture served by the combined waste & vent before you can tie the vent into the stack.