In my 1951 laundry room, the laundry sink drains via a threaded tee into a copper line behind the plasterboard wall. The washer drains into the laundry tub, which I want to change to a standpipe.

Based on what I've read here and elsewhere and in the IRC, is the only correct option to open a section of the wall and install another tee for the washer, below the tee for the laundry sink?

That does create a wet vent for the washer (of a few inches) but should conform to the IRC, at least from my reading.

It makes for a lot of extra work, but I'll do it if that's the only right way. Just wanted to ask though to make sure there's no compliant option that involves using only the single existing tee.

Thanks.

Open up the wall. Your Laundry tub drain is probably a 2 x 1 1/2x 1 1/2 San Tee. If true, is there enough room to cut under the laundry sink tee and install a 2 x 2x 1 1/2 tee, if true it would be a legal install. If not enough room you will need to remove the 2 x 1 1/2 x 1 1/2 san tee and replace it with a new 2 x 2 x1 1/2 san tee and then install your existing 2 x 1 1/2x1 1/2 san tee on top, for your standpipe. You will need to remove enough of the vent above the Tees and reconnect with a coupling. You can wet vent when the distance between the tees is less than 5 times the pipe diameter. Unless the new code changes this due to the volume of newer clothes washers, this would be the case. But be safe and speak with the local authorities to comply with the code.