Chris Green wrote:

Are there special planning permission rules applying where one wants
to build a house for a fmaily member (child) on one's own land? We
have a small-holding in an area where adding new houses is basically
not allowed but a few people have added houses for family members
and/or workers.

You can get special permission for an agricultural workers' dwelling,
but you have to demonstrate a suitable need.

I get the feeling that councils are very wary, because after a few years
people try to wriggle out of them, and revert it to a "normal" house
with no strings attached, so I wouldn't see it as an easy way to get
permission you wouldn't ordinarily get.