Having long endured noise (def: unwanted sound) in our house I can sympathise with your situation. I can't offer much help, I'm afraid beyond the advice to get a sound level check done yourself. Maybe your neighbour would share the cost. However, if the level is lower than the minimum specified by building regulations, it doesn't really help your cause beyond being an independent reading and not one "paid for " by the building company.
I would check for yourself too what the building regulations actually say about minimum requirements regarding sound transference and not just take the builders word for it.
Measuring sound is a tricky business and in fact, the measurement tends to be an aggregate over a period of time, say one hour or twelve hours, meaning the peaks of the noises you hear e.g. laughing, hoovering, etc wont really register when combined with the quiet times in the same period. However, we know that it is these very peaks of noise which cause the problem.
There is insufficient legislation or regulations on noise in this country. Noise is a serious problem, one that few care about until it directly affects them.
Good luck anyway. Noise is such a nuisance.