Originally Posted by DrumEatDrum
It depends on how the building is constructed.
As Pocket says, it's not the noise, it's the vibrations from the kick pedal that go straight into the floor. And if you have rubber pads on a simple rack, the vibrations from that can go into the floor as well.
OP being in UK might not help, as some flats (usually conversions) have really thin and/or poorly isolated walls/floors.
The building where I live is pretty much all concrete, but the vibration from the pedal was going all the way to the flat downstairs over the wall OPPOSITE the td4 kit.
I suppose the vibration "travelled" through the floor and then moved down on the structural wall.
In my case, I solved with the suspension platform (I am using foam underneath and not tennis balls) and some cooperation with the neighbours to understand when the vibration acutally stopped.