Unfortunately, a lot of info on chickens' space needs are taken from the legal minimums for commercially-kept birds, who are intensively managed in ways that backyards mainly cannot duplicate and probably wouldn't want to duplicate.
Here's a excellent discussion about why those numbers above are *guidelines* rather than hard-and-fast rules: https://www.backyardchickens.com/articles/how-much-room-do-chickens-need.66180/
Where, in general, are you located?
If you're in a warm climate you might be able to turn your under-deck run into an open air coop by putting some kind of roofing over a section and adding some kind of weather-shelter to the windward side of the enclosure.
Here's a excellent discussion about why those numbers above are *guidelines* rather than hard-and-fast rules: https://www.backyardchickens.com/articles/how-much-room-do-chickens-need.66180/
Where, in general, are you located?
If you're in a warm climate you might be able to turn your under-deck run into an open air coop by putting some kind of roofing over a section and adding some kind of weather-shelter to the windward side of the enclosure.