The additional building is an unknown quantity, we hear (6)
I believe the answer is:
annexe
'the additional building' is the definition.
(I've seen this before)
'an unknown quantity we hear' is the wordplay.
'an unknown' becomes 'ann' (I can't justify this - if you can you should believe this answer much more).
'quantity' becomes 'X' (X is an example).
'we hear' indicates a 'sounds like' (homophone) clue.
'x' sounds like 'exe'.
'ann'+'exe'='ANNEXE'
'is' is the link.
Can you help me to learn more?
(Other definitions for annexe that I've seen before include "Additional room etc for house" , "Extra wing (on building)" , "Attached (set of) room(s)" , "Extra piece of building" , "Building joined to a larger one" .)