It does indeed depend upon when it was done. Regs changed in the 80s. Prior to that it could be like my house, ie floorboards up and plaster down with only fresh air between. That's fine in one dwelling but no good otherwise.
I always found flats to be expensive relative to terraced housing of the same size.
I've got some sort of semi autistic/OCD issue with noise hence I'm limiting my search to properties built in certain ways- like semis with the hall and stairway on the adjoining side and terraces where one side is the ginnel and the other side is a stair/hall way. Flats are OK if they're like my current one, but even this can be annoying, especially now 18 kids have moved into one of the other flats.