Standpipe questions, venting and drain size
I'm installing a front-loading washer and dryer in a room in my basement which is adjacent to a full bath and I've got a couple of questions about venting and drain size.
I'm installing a standpipe and washer outlet box. From what I've read, that standpipe and the drain back to the main should be 2". The problem is that the existing drain line is 1 1/2". When I tackle the bathroom, I will replace the whole run with 2", but that won't be until next year and I need to get the washer running in the meantime. Should I go with a 1 1/2" standpipe so the pipe is the same all the way back, or a 2" standpipe and reduce it? If so, where's the best place to do it? I have access to about a 6 foot horizontal run.
The other big question is the venting. I've included a diagram which is hopefully pretty clear. The toilet and shower seem to be on a different line than the sink and standpipe. The main drain to the street is on the other side of the house. There's a horizontal pipe under the slab that runs over to that. I'm assuming the toilet, tub, and stack in the corner all feed down into that.
The only fairly easy way I can think to add a vent close to the standpipe is what I have pictured in blue. Because of the window and a soffit on the wall, it's got to make the two bends. I'm also not sure if it's okay that it's running into the upstairs sink drain.
The other thing I've thought of is using an AAV next to the washer. My understanding is that this has to be accessible, so I'd put it behind a panel or door or something.
Any help or advice would be much appreciated.