Extension to building (6)
I believe the answer is:
annexe
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">extension to building</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for annexe that I've seen before include "Supplementary part" , "Small building added to another" , "Extra piece of building" , "Added section of document" , "Extra accommodation block" .)