There is someone here on BYC that did similar. It was/is quite a large thread. I want to say it was in the mountains of TN, but I don't remember the name of the thread or the member who started it. Maybe try a string of search words?
I remember them having to have the "barn " area excavated & leveled, trenches for water drainage, etc. Lots of pics as they went along. After company installed building, I believe she & her spouse built the individual pens & storage room inside the building. I think they may have put some outdoor runs on as well...
I'm pretty sure they did install a fan system along w/ more ventilation then a regular barn or shed would have.
They also went over things that were wrong from the builder & things they would do differently.
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Check w/ Morton. They do custom builds & pretty sure wouldn't be difficult. Most other builders could do same.
Be careful about builders, scams have popped up w/ horse barn builders. Know or heard of several in past 10 years who've put down major deposits, work started & then builder or subcontractor take off w/ funds. Places/builds left a jumbled mess...
Another option is a carport build. They have open carports that you can use & build your own interior. Some have various closed in areas w/ a door. The price in our area used to include the cost of delivery & set up. Usually you have to level the area & may have to do some type of foundation.
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Edit to add - I found the big Barn thread...
Thread 'Building New Coop/Barn...Phase 5 Great Barn Build, OCCUPIED! 3/6/16'
https://www.backyardchickens.com/th...e-5-great-barn-build-occupied-3-6-16.1023932/