Admission offer deadline extension
I think a very short email saying you are going to wait until the deadline for accepting the offer, with no further explanation, would be enough. You are not doing anything wrong and April 15th is the original deadline.

Perhaps it wouldn't even be necessary to explain anything. However my suggestion is that it is always better to be wrong on the well-mannered/ cautious side, than thinking "it is not necessary" and then find out it actually was.