Within the Bureaucracy menu there is an option to purchase "Land Expansion" under the finance section. This will allow you to select land either Above, Underneath, to the side or across the road from your current prison and purchase the land. You can purchase up to 2 extra pieces of land, but you only require one for the achievement.
You must have "Finance" already researched and an accountant on site with her own office to allow you to purchase land, and the value of the land will vary depending on size/difficulty etc.
"Land Expansion" under bureaucracy itself costs $1,000 so it's not too expensive to learn pretty early.