The house was large, constructed of white clapboard and sections of stone with black doors and shutters and no fewer than four stone chimneystacks. A broad front overhang supported by rows of elaborately milled columns offered up a fine porch with rocking chairs, sturdy tables, hanging plants and an upholstered swing anchoring one end.

I understand that the front of the house was broad and was supported on columns and it was a nice porch with all this stuff on it including a swing. Is that right?
Yes. The description of the house implies that there was an overhang - a section that sticks out from the building - at the front of the house which was broad and there is a porch underneath it.

Undearneath it?

I thought the porch was on it. Not underneath? Are you sure?
Site Hint: Check out our list of pronunciation videos.
Pretty sure it would be underneath, based on the description indicating hanging plants which would hang from the overhang ( sorry about all the 'hangs' . . . makes this sentence sound awkward!!) and the upholstered swing which would also be attached from above.

Also, typically a porch is a off the main floor of a house, an area that is supported above ground level is referred to as a balcony or a deck