If you're a registered farm you do have to apply for permission now,
unlike
about ten years ago, but provided the new building is in the curtilege of
the farmhouse and not on the actual farmland they have no grounds to
refuse
permission. Same with cowsheds.

But not with stables.

Much more complex than that these days. Locally several farmers have had
applications turned down in the last year because there was no proven need
for the buildings.