You're figuring base minimum (100sq ft for 25birds), just understand BIGGER the better. What about their run? Or are you expecting to free range daily, weather permitting? If you're putting the nest boxes & roost upstairs, you're taking space away from your footprint. Also depends on the breeds you have/keepin. It may work to "increase" your bird capacity but be prepared for possible squabbles & having to enlarge your space
Chicken Math is very difficult to control. I have a 8 x 12 x 7 Chicken House (coop/run combo) giving me a 96sq ft space & mine are confined. I allowing myself no more than 6 (4sqft/coop & 10sq ft/run = 14sq ft per), however find that 4BO is all I'm comfortable keeping. I've also had 5 Wyandottes fit comfortable and no squabbles. I want my chickens happy & comfortable, avoiding squabbles & keeping egg production up.