What are the tax implications, if any, of selling part of my garden?

I am currently discussing selling part of the garden of our only residential property and would like to know if tax is payable on the proceeds.  The land would probably be sold to a builder for the construction of two semi-detached houses. 

The capital gains tax exemption for principal private residence includes grounds not exceeding half a hectare (approximately 1 1/4 acres), or such larger area as is appropriate to the size and character of the house. If some of the land is sold, including for the purpose of building plots, the sale is covered by the exemption so long as the land was enjoyed as part of the garden and grounds, and is sold before the house is sold or at the same time the house is sold (but not after the house is sold).