The restaurant on site will also be extended to 400 seats.
The number of free parking spaces available on site will be increased by 100 bays, with additional disabled parking.
The main contractor for the extension is Maher Millard and the architect is Evolution Design and work on the extension began in July.
Owner Richard Pennell told HW: "We're expecting hopefully to turnover £5m-£6m plus concessions after this, up 50 per cent. We will add a foodhall and 1,500 sqm of internal space and 3,300 sqm of canopy covered area Velo-type, like open skies for outdoor plants, providing more of what we're doing at the moment."
He added: "The development of the centre is very exciting for us and will provide an improved shopping experience for our customers with a wider range of products. However the heart of the business will continue to be our love of plants, gardens and gardeners."
The development will also include an outdoor dining area and more concession shops.
Pennells hopes to have the work completed in February 2015.