Depends on the type of house!

At least 1 1/2 - 2ft down from the ceiling to give enough head room and higher than the nest boxes.

Hens can jump a foot or two easily so somewhere in that you should be able to plan a height.

Remember they have fairly large feet and like a widish flat bar with rounded corners - I use a piece of 2x2 which seems ok.

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The weeks and the years are fine. It's the days I can't cope with!