I have a slab house and am updating a bathroom which will include a washer/dryer in it. I have a 1 1/2 inch drain available for both the sink and washer to use. The washer is a high efficiency/low water model, and the faucet uses 1.5 gpm. Should I be OK to put both drains into the 1 1/2 inch pipe?

Neither the sink or the washer should discharge into a 1 1/2" drain line. The newer codes mandate a 3" line for the kitchen and the washer should tie into that with a 3 X 2 wye or combo. Do it right the first time and you won't be going back the second. Good luck. Tom

Well, with it being slab, it is not in the budget to go ripping up the concrete, and the washer drains into that 1 1/2 inch now. There will be no inspection on the project and it just needs to work, not be up to current code. Being an electrician for 10 years, I am well aware that most codes are over the top and not necessary for things to actually function properly. It is only a small 1000 sqft single bath dwelling, what issues might I run into if I proceed?

well, with it being slab, it is not in the budget to go ripping up the concrete, and the washer drains into that 1 1/2 inch now. there will be no inspection on the project and it just needs to work, not be up to current code. Being an electrician for 10 years, i am well aware that most codes are over the top and not necessary for things to actually function properly. it is only a small 1000 sqft single bath dwelling, what issues might i run into if i proceed?

I see your problem. If there's no backups from the washer then you should be OK. Good luck Tom.