What if you want to sell off some of your garden for someone else to build on? Will the exemption apply? In simple terms it will if you continue to own the property with the rest of the garden and the total original area was within the half a hectare limit.
Where the total area exceeds half a hectare and some is sold then you would have to show that the part sold was needed for the reasonable enjoyment of the property and this can clearly be difficult if you were prepared to sell it off.
What if on the other hand you sell your house and part of the garden and then at a later date sell the rest of the garden off separately, say for development? Then you will not get the benefit of the exemption on the second sale because the land is no longer part of your main residence at the point of sale.