Not hearing next door, a bathroom or simply being able to listen to music in one room and the tv in the other all rely upon good sound insulation or separation. Acoustic design can get very complex very quickly, and anything out of the ordinary will likely require specialist advice, design or construction.
Part E of the Building Regulations explains and gives minimum values for most types of new build and conversions. These values are expressed as simplified numbers measured in decibels. (But unfortunately it becomes more complex as there are decibel ratings in laboratory conditions , ratings with a low-frequency bass weighting, on-site testings and others.)
Generally extensions to a single dwelling do not need to meet any specific criteria, but rather it focuses upon new build, conversion or change of use (eg office to dwelling etc). If works would harm a Listed Building, then the requirements can be relaxed. As you would expect separation between separate dwellings or functions demand better performance than between rooms within the same occupation. On conversion of a building to flats for example between separating floors, you would likely need an acoustic floor laid over the existing floorboards (or new), acoustic quilt in the floor zone and a new ceiling below as a bare minimum. All the joints also need to be sealed to be effective.
There are a couple of facets to achieving good sound separation; what the floor, ceilings and walls are made of and the site workmanship quality. Airborne sound separation in layman’s terms can be considered to be a bit like imagining the room is filled with water to the ceiling – any gaps around sockets, lights, pipes, keyholes etc will all leak sound, possibly ruining your quality wall. Reducing impact sound is generally about isolation, mass, and a change in materials, all in the name of absorbing the sound energy. Each layer and change of material will take energy from the sound waves as the material vibrates the next material. Think how easily and far sound travels along a tapped pipe, compared to tapping a much shorter but sandwiched hard-soft-hard construction.
Sound ratings for airborne and impact are dealt with separately but most noises have an element of both.
On high demand rooms like theatres or recording studios it is not uncommon for acoustic consultants to help detail the construction. Places like purpose-built chain hotels often have rooms built as independent ‘boxes’ isolated from the structural frame with rubbery materials to avoid transmission of noise. In the home environment, for a serious soundproofing a secondary skin of wall or 2nd ceiling as a service zone to house all the services and sockets etc is the most effective way to avoid sound-leaking penetrations. These twin-wall systems have to stand independently from each other as separate walls and you can easily see they double the width and the cost. Sound can also ‘leak’ around junctions where walls, floors and ceilings meet, as well as flanking sounds looping around openings such as open windows etc.
As you can begin to appreciate it is a complex science and it would be foolish for an architect to profess any expertise other than general awareness. The following 3rd party page offers a good insight into what is required from buildings and the difference between the ratings, decibels and what to expect should your building require on-site ‘pre-completion testing’.