Hi Craig, For the dimensions of your coop, each chicken will need 12ins of perching space, bantams (like silkies) will need a bittle less (about 9 ins).
For the run, each chicken will need a MINIMUM of 1sqm, preferably 2, and if they are in a permanent run, then a minimum of 3. Chickens are terribly hard on grass, and within days it will all disappear! (I have 7 pullets - 4 bantams and 3 l/f) in 100sqm and ther is hardly a blade of grass to be seen!
Given that your max run space will be 4ft x 18ft, and you will need space of feeders, drinkers, a dust bath and perches, I would recommend a maximum of 5 hens, if you are getting 2 silkies, or 3 hens if you are getting all farge fowl.
With silkies,; you have to remember that they are bred to be great mums, therefore, they can be a bit spotty with the egg laying, and they are so often broody (though I understand them to be very friendly, not that I have any personal experience with silkies though!)
A hen house doesnùt have to be high. In fact if it is too large then your hens may have problems keeping warm enough as thier body heat won't warm the air in the house. Just be aware when you build it, that you need easy access to all corners for cleaning purposes, and if it is too great an area you will have issies with not being able to reach the far side!
Good luck and happy chickeneering
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