I’m going with fandom urban legend myself. That section is way too big to be an addition by production, especially on their paltry budget. They’re not pouring concrete for the base, and building walls, and roofing it. I’d say it’s an original part of the house. If anything, they may have simply removed the door for the camera, thus creating the legend.
All great points. I’d also like to mention, what would even be the point of adding this elaborate addition? The only time they even filmed back there was for a single shot and it was only to walk through the back door to gain entrance to the house. I’m certain this was an authentic addition added by one of the owners over the decades the house stood there.
I contacted Kenny for more info. According to him the original house plans didn’t include a kitchen. This could very well be why it was added to give Michael the ability to get his knife.
Of course the other option being that it was added at some point and removed between filming and the purchase/move of the house to its present location which is entirely likely as well.
I was always under the impression it was an add on that was done before the film and when Margraves moved the house he didn’t take the addition just the historical original house.