While some might refer to the top part of the house, where hay can be stored (and serve as insulation), as the grange - and this was the case in a village in Ardèche I stayed in many years ago, but there the houses were on the side of the hill, with ground-level access to the grange at the back while the main front entrance was a couple of storeys lower, so it was almost like two different buildings - ... a grange is generally exactly the same as a barn, i.e. a separate building.

Quite why they draw this analogy though, I don't fathom, unless it's something to do with the architectural style of the building?

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Note added at 1 hr (2009-06-22 10:35:02 GMT)
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If you look at the picture of it here
http://www.lescarnetsdudesign.com/?cat=6
it does bear a certain resemblance to a barn.

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Note added at 1 hr (2009-06-22 10:38:16 GMT)
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And seen from the inside, the 9m-headroom space at the top reinforces the notion of "barn".