burgerproduction
I've seen many projects for raised/floating floors for studios, but this seems to be to prevent drums or noise from within the room leaking out.
What prevents sound getting out will also prevent it getting in. The problem is that low frequencies in particular are really hard to keep out. Especially if a problem in the flat below is something like a hi-fi with speakers on the floor that in turn connects to the walls and the sound then travels through them and up to your flat that way as well as directly through the floor. Steel joists can also be excellent sound transmitters through a building.
The noise can almost certainly be kept out, but expect to lose quite a bit of height in the flat because the floor will end up raised several inches (at least). The walls might also need several inches of soundproofing added to them.
Which in turn means floor loading may become an issue, because effective low-frequency blocking requires non-resonating/vibrating mass to be effective. And depending on the materials used there may also be fire prevention implications as well.
Reducing transmitted noise by a few dB often isn't too difficult. But reducing it to an acceptable level might be much harder, and the more reduction is needed the harder it gets. The law of diminishing returns sets in pretty quickly, hence the "room inside a room" approach to studio building.
One option might be to lay wooden joists on top of the floor. Fill the gaps between them with the heaviest grade of rock wool, then suspend another floor on top which is isolated from the joists by weight-supporting rubber blocks such as Auralex U-Boats.
https://www.auralex.com/p...u-boat-floor-floaters/ Assuming the floor can support the weight of course.
Things like electric sockets might need moving depending on how close to the floor they are in the first place, and the total cost (and weight) will obviously depend on the size of the flat. Then there are the walls to consider - if sound is travelling up them from below they'll need treating as well. And the doors would obviously need adapting if the floor height is raised.
All in all, quite possibly a big and expensive job.
And maybe the flat's selling at the price it is because of the problems with the neighbour below it....