Why does my kitchen sink gurgle and smell like sewage when the washer drains?

That gurgling and subsequent smell is the textbook definition of a siphoned p-trap. When your washing machine discharges, it pumps out a high volume of water very quickly. If your plumbing system isn't vented properly or if the pipes are undersized, that massive slug of water creates a vacuum as it rushes past the kitchen sink's connection point. Since the air can't pull from a vent stack like it's supposed to, it sucks the water right out of your kitchen sink's trap to 'breathe.' Once that water seal is gone, there's nothing stopping the **sewer gases** from flowing directly into your kitchen. You mentioned that the addition was done poorly without a shower trap, which strongly suggests the installers probaly skipped the venting for the kitchen and laundry too. Modern washers realy need a **2-inch drain line** to handle the force of the pump. If you're running on an old 1.5-inch galvanized line, it’s likely restricted by years of lint and grease buildup. That restriction makes the vacuum effect even stronger beacuse the pipe is essentially running full of water with no room for air. To fix this, you should first check the roof to see if there’s even a vent stack above that area of the house. If there is, it might be blocked by a bird's nest or debris. If there’s no vent at all, you might be able to install an **Auto Air Vent** (also called an AAV or Studor valve) under the kitchen sink. You’d install it on a tee after the trap but before the pipe goes into the wall. It’s a one-way valve that lets air in so the vacuum doesn't suck the trap dry, but stays closed so smells don't get out. Just keep in mind that an AAV is often a band-aid for poor layout or undersized piping. If the laundry line is partially clogged, getting it snaked with a professional-grade cable could also alleviate some of that pressure. But definitly don't ignore it; those gases aren't just gross, they can be a health hazard if they're constantly venting into your living space.

Man, if they skipped a trap on the shower, you've got a real DIY nightmare on your hands. I'd bet the washer and sink are tied in way too close together without any proper venting between 'em. Check your washer standpipe height too—if it's too low, it'll just hammer that line even harder. Since you're remodeling soon anyway, grab a cheap camera and look behind the drywall. If they cut corners on a shower trap, there's no telling what kind of mess is hiding in those walls.

I agree with everythign said above, especally about the AAV being a solid temporary fix. One thing people often overlook in these scenarios is the standpipe for the washer. If the previous owners did a hack job on the shower, there's a good chance they didn't install a proper P-trap for the washing machine itself. If that's missing, the vacuum isn't just pulling from your sink; it’s creating a massive air turbulence throughout the whole branch. Definitely check if that washer drain has its own dedicated trap before you start cutting into the sink lines.

I'm not totally sold on this being a venting issue just yet. Have you ruled out a simple partial blockage further down the main line? Everyone jumps to siphoning, but if you have a soft clog of grease or lint downstream from where the kitchen and laundry meet, that slug of water from the washer is going to back up and push air out the sink trap before it drains. It sounds like a vent problem, but I've seen 'gurgling' that was acutally positive pressure pushing air up, not a vacuum pulling it down. Maybe try running a snake thru the cleanout before you start cutting into the drywall to add an AAV?