Footer under fireplace/chimney

Can someone tell me whether the footer for a chimney/fireplace is the dimensions visible above ground or does the footer extend a foot as it would with a wall. I live in the Northeast where the frost line is 42". The footer will be most likely pinned to ledge. We haven't decided on the chimney specifics (it will be built as a later point in time)but feel it can match the existing one on the house and I just need to tell the concrete contractor the dimensions of the footer.

Any thoughts would be appreciated. Thanks.