I think you are trying to accomplish the same thing I was, check out this question where I got my answer from or see the code below:
CGRect oldFrame = WebView.frame;
CGRect newFrame =CGRectMake(oldFrame.origin.x, oldFrame.origin.y, oldFrame.size.width, oldFrame.size.height-100);
[WebView setFrame:newFrame];
Also remember to uncheck the Auto Layout feature within Interface Builder / Storyboard.