The Google office I worked in had this issue and they didn't do **** about it either, claiming they were working on it as it persisted and grew worse over the months. You know it's bad when you have to swat drain flies off your **** whenever you go to pee
Sad thing is there isn't really much you can do. The flies themselves are extremely annoying, but they quite shockingly don't spread disease. And killing/trapping the flies accomplishes nothing, because they're not breeding in the residence--they're breeding in the pipes and sewers, where gunk always builds up over time. The larvae feed on the bacteria in those pipes, which is why it's amazing they don't spread disease. They are however
insanely hard to kill. You can't drown them,
bleach and draino won't even kill them, and your only option is to
thoroughly clean and decontaminate your entire pipe system (and sewer/septic tank) with lots of chemicals and a cleaning brush. Our entire building--which is about 100 apartments--has the **** things, which likely means the problem is in the freaking sewer. Which is the city's problem, and well, our lovely little section of the Kansas City area is CHEAP. They won't even repave our road--which is littered with cracks and potholes--and our apartment complex has to plow it themselves because the city refuses to do so. So yeah, this problem isn't going to go away any time soon.