"The term 'built in' makes me think that they are attached to the wall, however are
these things just generally built to fit the space and left free standing ?? "
[..]

ours (I mean the previous owners) were damn ugly Sigma3 units and where screwed to the
back and side walls, the floor and ceiling joists (god knows why).
taking them out was a breeze, just undid the screws and took each unit out separately,
then through them out the 3rd story bedroom window into the garden (read waste land)...
that was the fun bit (c;

now for the ****ty bit... making good the mess that was the blown plaster damp wall
behind them.

LJ