Attic

Then what was the attic-like room shown in the movie?

It's a crawl space

There's metal railing in front of the top windows. The attic is more like a small loft space. At least little bit of storage area would be needed in any house, especially a glass one.

It absolutely didn't exist and couldn't possibly fit into that house. Not even as a 'crawl space'.
A child could easily stand up in there, it had a decent height and obviously Keanu was reaching it from a ladder.
There are shots of the house from nearly every angle. This 'attic' was not there.
Now, the question is: why would the writers and the director allow such a silly mistake? Just make it that the box is in one of the kitchen cabinets! How hard is it? o.O

That was a goof. That attic area that Jack entered was huge. They showed all sides of the lake house during different parts of the movie. There was no place where the attic shown would fit.