I plan on upgrading them to a nice greenhouse in my back yard. We have an acre. I want to take my time to make it perfect against predators. How long do you think I have? The kennel is 6x10. I have a dirt bath area, roots, vegi treat bag...
I would probably try and build something bigger by the time they reach full size. Or maybe just keep 5 of them. Or do what I suggest below.
There is a recommended square footage per bird for runs and coops, you can probably find it on this site or elsewhere online. And then most people say you should double it because these official 'recommended' calculations are for commercial birds being reared intensively on battery farms and which live short lives. Backyard keepers have different aims, which is to provide long, happy, fulfilled, harmonious and interesting lives to their chickens.
Not enough space can lead to stress and discord. You can increase what you already have by using vertical space, as in putting things they can jump on and perch, like an old chair, a rope swing, some outdoor perches, an old tote lid on a brick stack, this is what is called a home made chicken gym. It doesn't need to be costly or fancy. Some old wood can easily be repurposed.
Also if you build a new one, I would recommend a wooden shed and an open sided run rather than a greenhouse, as they get super hot in summer. Even your plastic side sheeting now could be just wire. So many coop and run ideas on this forum. Take a look. What you have now isn't bad, but you could improve it.
I maximise my limited space with various chicken gym arrangements, free ranging and only keeping bantams!
I wish I had an acre!