I've had problems with this, and I am no expert but it is more possible to
get councils to agree if you are prepared to add some species of reversion
clause (non-technical use of language here) such that in so many years or
on
change of ownership the land reverts to agricultural use. I don't think
this
is advertised but it certainly does happen.

Certainly there is a mechanism to do this. One of the prime reasons for
restricting extension of gardens into agricultural land is that there is a
presumption in favour of development with the domestic curtilage. Therefore
if the garden is extended without restriction it could open the way to more
houses being built on land that would not otherwise be permitted. Usually a
legally binding agreement is required that prevents the land being separated
form the existing house.