Hmmm i dunno how but maybe the secret basement could attaully be aboth ground but made to look like its lower down ?
EDIT:
"Incidentally, the Big Brother house currently stands on the water tank that was used for the film Moby Dick way-back in 1956!"
http://www.freewebs.com/elstreecalli...iefhistory.htm
Sorry to double post but i found that it is build on a Water tank according to that unofficial elstree website
http://www.elstreefilmstudios.co.uk/
On this site on the picture you can see one of the bb studios
Just thought it was interesting