If you are making a classic 'toblerone shaped' ark, 42 inches wide, it will be about 3ft high, plenty enough headspace for chooks. Ideally the 'indoor' bit will be no more than a third of the total. If it gets moved a fair bit, and they get extra freedom when practical, that will do for 3-4 big hens, or about 6 bantams (even more if they are really small banties). They might be a bit happier in a 'box-shaped' ark (and probably easier on the DIY skills) and if you make it 42" high (if that is the size of timber you have handy), the house can have open space underneath, in which case it can be a bit more than a third of the total 'ground area', and the top half of that space. That would be comfy for 4 big hens. (my first henhouse was not very different from that description, and we had 6 bred-for-battery hens in it, let out when we could).

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