I am currently converting a garage in to a kitchen and wondered if i could tap the sink waste pipe in to the downstairs washbasin wastepipe which is just through the other side of the wall, the concerns I have are A, the washbasin pipe is 32mm and B, this runs in to a boss which joins to the toilet waste any advice is much appreciated.
The new kitchen sink waste will be 40mm which is larger than the 32mm basin waste. In my opinion coupling the two together could cause problems especially if the sink is a one and a half bowl or you intend to use a washing machine or dishwasher in your new kitchen.
The regulations say each should have its own coupling to the soil or drain and doubling up is not permitted.
If the stack is accessible can you not put a strap boss on it so it has its own connection?