Adding a Laundry Sink to Existing Washer Drain

[bradleonard]

I posted this question at the end of a very old thread (http://www.terrylove.com/forums/show...ht=#post277172) and didn't receive a response, I have an existing standpipe for my washing machine going into a 2" drain vented above with 2". The washing machine drain comes from the left in my picture.



I would like to add a laundry sink to this drain. My question is Washington Code specific as I live in Washington. Can I simply add a cross T where the washing machine enters this stack and drain both the sink and washer at the same height, or do I need to do a vent in the horizontal line come from the sink trap just as you have shown in the picture at the beginning of this thread?