Architectural afterthought
I believe the answer is:
annex
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">architectural afterthought</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for annex that I've seen before include "Add to one's territory" , "Attach(ment)" , "Append" , "Appropriate" , "Join - add" .)