How to soundproof your property
The downside of soundproofing walls is that you will lose space as you have to install a secondary dry lined partition wall. Five centimetres is about as little you can lose while still significantly reducing noise levels.
The secondary wall should be made from metal resilient furring channels, soundproof plasterboard and other soundproofing materials.
Furring channels are special metal dry lining frames designed to minimise sound transmission. They do this by having minimal contact with the existing wall.
Screw/nail resilient furring channels to your existing wall.
Glue polyester sound absorber sheets to the wall. Note: they have to be thinner than the furring channel, as you do not want them pressing against the new plasterboard.
Fix the first layer of soundproof plasterboard to the furring channel.
Fill the gaps between the plasterboard.
Glue a layer of visco-elastic membrane to the existing new plasterboard.
Fit a final layer of soundproof plasterboard.
Skim and make good (e.g. replace the skirting board).