Nobody asks about acoustics at briefing stage. Everybody asks about them eighteen months after moving in.
Where the noise comes from
In a modern house with a heat-recovery ventilation system, almost all of it. Duct-borne noise travels between rooms with nothing to stop it unless attenuators are specified at every branch, which adds about 1.5 per cent to the mechanical package.
The rest is mass
Concrete floors help enormously; timber floors need a floating screed or they will telegraph every footfall to the room below. Both decisions have to be made before the structure is designed, which is why acoustics belongs in the first meeting rather than the last.