“It sounds utopian, I know,” says Daniel Haime, standing over the map of Serena del Mar, the city he is building on the Caribbean Sea. He plans to transform a 2,500-acre site outside Cartagena, Colombia – which his family bought in 1968 – into a lively metropolis. There will be a world-class hospital, low-income housing and state-run schools, a marina and bike trails, a luxury hotel and waterfront dining. One day, vaporetti on the lagoon could connect Serena del Mar to downtown Cartagena.

It seems like an auspicious time to build a new city in Colombia. In 2016, the government and the FARC rebel group signed a peace deal ending 50 years of conflict. A cautious optimism buoys the economy. Bogotá and Medellín, the country’s largest cities, have become models for smart urban design. But not Cartagena. It may be one of the fastest-growing cities in Colombia but, according to Nicolás Galarza Sanchez, a research scholar at New York University’s Marron Institute of Urban Management, it is also a case study in how not to expand. It has spread in a fashion typical of South America: high-rise condominiums for the rich have sprouted along the waterfront while poor settlements made up of squalid, informal dwellings have grown in areas prone to flooding. Galarza and his colleagues are working with the Colombian government to help manage growth in 100 cities across the country.

Serena del Mar, seven miles up the coast, is being developed privately by Haime’s company, Novus Civitas, and will absorb some of the city’s population growth. In 30 years, it could be home to 100,000 people. The master plan has been drawn up by EDSA , an American firm which specialises in sustainability. Their design tackles two big challenges: how to build a city for both rapid population growth and climate change.

Vertical fins The business school for the University of the Andes

Many recent developments around the world bear the influence of Le Corbusier, who advocated destroying historic downtowns and replacing them with superblocks, or “towers in the park”, with little sensitivity to the local environment. Haime’s vision is for a green city, where nature guides the design. He was inspired by the work of Ian McHarg, a Scottish landscape architect who pioneered an ecological approach to development in the 1960s.

Among the architects for Serena del Mar is Brandon Haw, whose firm is behind four buildings, including a business school for the University of the Andes. “What you learn from history,” he says, “is not style but how people put buildings together to mitigate extremes of climate.” Just like in Cartagena’s old city, where streets channel the wind and homes are built around courtyards, his university building captures the northerly breeze and funnels it towards a pool in the centre, which helps to cool the space. Vertical fins, a modern take on colonial shutters, protect the façade from direct sunlight.

The city’s commercial centre will be built around a dredged canal and streams will course through residential areas. Three-quarters of the site will be parkland, protecting estuaries, mangroves and saman trees. Channels have been opened up so that seawater can refresh the mangroves. When it rains the water flows through valleys, which are no good for housing but excellent for golf courses, where the irrigation will keep the fairways green. The buildings’ foundations are elevated well above sea level to protect against rising waters.

Given Colombia’s extreme income inequality, it is the planners’ sensitivity to social issues that makes Serena del Mar such an unusual project. As in cities across Colombia, many of Cartagena’s residents have fled violence in rural areas. Decades ago, a group of refugees from the countryside settled on a beach that is now part of Serena del Mar. Landowners have a legal obligation to agree with those living on their land how it will be developed – a condition that is often disregarded. But having signed a charter with government officials present, Haime has promised that Serena del Mar will include accommodation, education and jobs for these people. Their homes will stay where they are. When they need extra room, there will be affordable housing nearby.

Construction is already underway. The business school will be completed this spring; students will arrive by the end of the year. Within 18 months, families will settle into apartments, buses will pull into the transit terminal, and an airy 400-bed hospital – designed by Moshe Safdie and consultants from Johns Hopkins Medicine International – will open, accepting patients on Colombia’s universal health-insurance programme.