How much of that is US-specific? According to defra:

Stocking rate in the house is as follows: ... Chickens = 13 birds but not more than 27.5 kg live weight per m²;

"the birds have had during at least half their lifetime continuous daytime access to open-air runs, comprising an area mainly covered by vegetation, of not less than:

1m² per chicken or guinea fowl (in the case of guinea fowls, open-air runs may be replaced by a perchery having a floor space of at least that of the house and a height of at least 2m, with perches of at least 10 cm length available per bird in total (house and perchery)).

2m² per duck

4m² per turkey or goose

source