It also secured retrospective planning permission to use parts of its land for growing plants, unloading lorries and for additional parking.
The application was submitted in 2008 and was developed by Pegasus Planning.
Rutland Garden Centre director Chris Hamilton said: "This will double business. We’ve been trading out of a 20-year-old garden centre shop with a flat roof.
"This has been going on a long time. Other garden centres are expanding and now we can move on and compete with them at the level we need to. This is massive for Rutland. I’m well pleased."
The building for the new sales area will be 43 metres by 12 metres, and will replace an outside sales area.
The centre is on the site of Rutland Village, which includes a farm shop, café, Cotton Traders, Maidenhead Aquatics, Perky Pets, Yeomans Outdoors and conservatory, shed and flower outlets.