The 400sq m extension is a steel construction, with part glass and part PVC covering. It creates a new entrance to the store and is mainly used to house plants.
Malcolm Scott consultants was working with Pontarddulais to design and build the extension.
Pontarddulais owner David Evans said: "My planteria has been greatly improved by this undercover extension. It gives me an opportunity to sell during adverse weather conditions."
He added: "The development gives a warm ambience that customers are happy to dwell in for longer, spending time on choosing plants that perhaps they wouldn't have done if they were standing outside in poor weather conditions.
"I would say it's taken the centre up a couple of notches in public perception. It's given us a nice modern feel in a 45-year-old garden centre."
Pontarddulais has more plans for improvements in the next 12 months, including recovering the patio area outside the restaurant. It also plans to open a 3,000sq ft outdoor living centre that will stock garden furniture in an on-site building that used to house a pet shop.
Evans said: "I've noticed that the numbers of repeat visitors are going up, with customers returning on a more regular and frequent basis. The response we've had from the extension has been positive. People are really enjoying it."