A field hayrack moved around regularly or put on an area of hard standing, with hay available at all times, should keep them fit. If the ground becomes very poached, however, it might be worth bringing them in for a while. Housing is a lot of work, runs the risk of pneumonia if the building isn't well ventilated and, depending on breed, could lead to some of them going stir-crazy after a while (thinking of my Badger Face here).