All I know is that I have a brand new (timber framed) house and I
can't hear a peep out of next door. Whatever the building regs are
you shouldn't be able to hear what would be considered normal every
day sounds, so if you can hear talking then there is a problem.

Yes - a pretty big problem I think. Building Regs are what I need on my
side though regardless of whether the average person agrees with you & I as
to what is reasonable or not.

From a DIY point of view, the only real solution to this problem is another
layer of plasterboard on the terraced side of every adjoining room (and all
the upheaval hassle that will cause) ... correct ??