How do the English laws and regulations governing 'designated
agricultural land' come into play when someone wants to convert farmland
into garden??

What actually happens if someone buys somewhere in the country with a
few acres, and starts turning a field previously used for agriculture or
grazing into a large garden? Maybe rearranging a few internal boundaries
in the process. Is this in contravention of the title deeds, or planning
regs, or do neighbouring farmers who could use the land for farming have
the right to object, or...? Just wondered how the system works.

Cheers,

Chris