You really don’t need much for a coop at all. When we bought our property it had a tool shed that we converted and it turned out to be way too big for our flock of 12. You could honestly use a large dog house. There are lots of plans online and Reddit has a very good chicken community with answers to any questions you might have.

All that to say: the minimum threshold for starting backyard chicken keeping is very low. I’d recommend jumping in as cheap as possible and upgrading later if you find you enjoy it.