Thanks all for the suggestions.

Andy,

I am seeking the answer to the customer's question: "how much space I need?" I presume, the customer KNOWS there have to be gangways etc., not only because of norms, but also because of his logistics (and I don't know the size of his carts, for example). So I think I don't really need to know an exact term for "D" (and I can tell the customer "add some space for the gangway/path/whatever", can't I).

-----------------------------------------------------------------

Gangway

----------------------+          +-------------------------------
:                     :          :
:    +-----------+    :          :
:    |     A     |  B :     D    :
:    |<--------->|<-->:<-------->:
:    +-----------+    :          :
:          C          :          :
:<------------------->:          :
+---------------------+          +

My limited understanding of English suggests that "clearance" is really a term for "B" rather than "C". And, the customer is interested mainly in "C" (he needs to know how many machines can he stuff into the shop), and also "A" (he needs to move the machines around and/or store them).
So "clearance" is a great word and sounds really great, but I'd like to ask you for one more, sounding similarly cool... ;-)

Thanks again,

Jan