Every time I use my washing machine (which is in the bathroom and connected to laundry tub) water comes up through the plug hole in the tub and overflows. I have to keep turning the machined off and wait for the water to recede. I live in a second floor apartment. It is a rental and only recently built. I had the builders out to look at it and they told me there was no blockage, plunges the sink and lef. It worked okay once, after they left then started backing up again. If the drain line under the sink is undersized, would it be expensive to replace.
Thank you
Julie

Call maintenance back, there IS a blockage. If there was no blockage they would not have plunged.