First thing to do is check to see if the existing wall is filled with any kind of mineral wool? If it has none then your best option is to strip the existing plasterboard, fill void with acoustic quality mineral wool, fix resilient bars, fix one layer of 15mm soundcheck plasterboard and seal all joints around the edges. Then fix a further layer of the same with the joints staggered to the first layer and seal the edges again. This will not be perfect but is the best you will get for the limited room you have.
It would also be worth pricing up the same option but using a composite mineral wool/sound barrier material. This would improve perfomance significantly but would add about £20/m² to the job.
If the wall is allready filled with mineral wool then you could consider just adding a further layer of 15mm soundcheck board on resilient bars. Probably not as good as the first option but will save a lot of work.
Ideally your neighbour would do the same job on their side, but that seems unlikely given what you said.
You should also check to make sure floor and ceiling voids do not pass through the dividing wall.
It would also be worth pricing up the same option but using a composite mineral wool/sound barrier material. This would improve perfomance significantly but would add about £20/m² to the job.
If the wall is allready filled with mineral wool then you could consider just adding a further layer of 15mm soundcheck board on resilient bars. Probably not as good as the first option but will save a lot of work.
Ideally your neighbour would do the same job on their side, but that seems unlikely given what you said.
You should also check to make sure floor and ceiling voids do not pass through the dividing wall.
