Completed in late 2013 at a cost of NZ$135,000 - half provided by City Hall - the 450sq m garden boasts azalea, lilies, maple trees, herbs and two small pavilions.
Company spokesman Bae Seung-San says staff use it to unwind and potential customers are taken to the roof as part of a sales pitch.
"When we have foreign buyers, we throw barbecue parties here, " Bae said.
The municipal financial support comes with a rider - any garden must be properly maintained and opened for public use within five years of its completion.
Since the project began in 2002, the city government has spent more than NZ$73 million helping to bankroll rooftop gardens, allotments or small ecological parks on more than 650 buildings around the city.
"We need more green, but don't really have the budget to buy the land for urban parks," said Bang Seong-Weon, a municipal official in charge of the Green Roof Construction program. "If you green the rooftops, land prices cease to be an issue."
Home to 20 per cent of South Korea's 50 million people, Seoul is a modern, thriving city with a population density nearly twice that of New York and eight times greater than Rome.
Largely destroyed in the 1950-1953 Korean War, Seoul was rebuilt at a time of rapid industrialisation and laissez-faire urban planning that resulted in a landscape of cookie-cutter apartment blocks and utilitarian office buildings.
In the past 10 to 15 years, efforts have been made to revitalise the city with varying degrees of success.
Bang is keen to highlight the economic as well as environmental benefits of the roof gardens, which absorb heat and act as insulators for buildings, reducing energy needed to provide heating and cooling in Seoul's freezing winters and hot, humid summers.
"And they improve the landscape, giving people a sense of the changing seasons," he added.
Although other high-density Asian cities have also turned to rooftop gardens, the scale of Seoul's programme sets it apart.
"I've never seen anything like this before," said Choi Da-Yeon, a 20-year-old student as she strolled among the garden's flower beds and trees with her boyfriend at Garden 5.
Han Moo-Young, a professor of civil and environmental engineering at Seoul National University, is a leading proponent of the rooftop movement.
At a cost of 200 million won provided by City Hall, corporate donations and his own money, Han laid a flower and vegetable garden that covers an 840sq m rooftop on one of the main campus buildings.
University staff, local residents and an organisation for disabled people tend the vegetables growing on one half of the roof, and the flower beds on the other half, which also host six beehives.
"We hold music concerts here and make kimchi together with students and residents. It's a community," Han said. "You see the bees and butterflies and you get that sense of being in nature."
While the rooftop project was designed as a co-operative effort involving government, corporates and individuals, the Seoul authorities are about to start work on a far larger undertaking to convert a stretch of abandoned 1970s highway into an elevated park.
The city has engaged Dutch architects MVRDV to design the Seoul Skygarden, due to be completed in late 2017, partly inspired by the success of New York City's elevated High Line.
In total, the pedestrian park spanning Seoul's main station will be home to 254 species of flora, as well as an nursery arboretum to provide cuttings and saplings that can be transplanted to other rooftop gardens around the city.
Exposed to strong sunlight, the elevated gardens need more care and watering, while taking a bit more effort to get to, acknowledged Seo Jin-Sook, who grows vegetables atop the university.
"However, looking at the beautiful scenery on top of the rooftop while gardening is a healing experience for me," she said.
CHECKLIST
Getting there
Korean Air flies from Auckland to Seoul, with Economy Class return fares starting from $1940 for departure before November 13 and between January 18-November 10, 2017.
