The expression secret annexe was created for the English translation of her diary.
It is pronounced ANN-x. That is the English word for it, in Dutch it was called the "achterhuis", the "back house". That is pronounced OK-terr-hois.
The leader was Otto Frank. He was in charge of making every decision.
Above Mr.Franks office were he used to work/own
The spelling "annexe" is chiefly a UK spelling. Correct in the US is "annex" (noun or verb).