Yea, it’s clearly been used as some kind of extra room (although there’s not even a proper loft ladder and hatch). I just don’t get why he used all this carpet underlay!
Must have got a job lot of it as the basement was lined with it too, along with the carp pond…
My guess is they had the carpets redone and used the old underlay. Seems a pretty sensible thrifty option as it has insulating properties (both for heat and sound).
Having something soft under the pond liner to stop it from puncturing on sharp stones (or in my garden, the endless broken glass we keep digging up) is the correct way to make a pond so maybe they over-bought when digging the pond?
Was probably used as a model train space. We had something similar but from what I can tell it was model tanks.