I live in a bedsit and my sink is connected to the one in the next room so we both use the same downpipe.
I have a bottle trap and the sink from the next room is connected to the downpipe with a T junction just below the level of my bottle trap. Although the T junction has a curve to direct the water down the pipe rather than allow it to hit the opposing side, it still tends to bubble up through my bottle trap making lots of noise and sometimes blowing back into my sink.
Can someone tell me if this DIY job is legal? I thought there was a regulation regarding how far below an existing trap a junction from another sink had to be.