To take an agricultural field into a garden opens it up to
development at some future date. you can see the opportunity to buy a
property on the village edge. Extend the garden by 30m into a filed,
then a few years later sell off the garden as a building plot, or
build a granny cottage on it.

Unlikely to work because the developement limit will not include the
extended part. That is not to say it has not happened but the planners are
quite alert to such scams today.