Reckoning it's an annex (8)
I believe the answer is:
addition
I believe this is a double definition.
'reckoning' is the first definition.
(I've seen this before)
'an annex' is the second definition.
(thesaurus)
'it's' is the link.
(Other definitions for addition that I've seen before include "Arithmetic calculation" , "Something extra" , "Mathematical procedure" , "Increment" , "Rider" .)