There's no mistaking Vann Molyvann's house. Partially hidden behind high steel gates, like most residences in the Cambodian capital, Phnom Penh, his concrete and red brick home supports a soaring, pagoda-like roof. It's a typical Vann Molyvann touch; testimony to the unique ability of Southeast Asia's greatest living architect to fuse European modernism with traditional Khmer design in an apparently seamless style.

Built in 1966, the house, on Mao Tse-tung Boulevard, stands in defiance of the homogenous box-like buildings that increasingly surround it. As investment from China and South Korea floods into booming Cambodia, the centre of the capital is changing rapidly, apartment and office blocks being thrown up in seemingly random fashion.

Phnom Penh is renowned for its beguiling blend of architecture: some of the most stunning modernist buildings anywhere in Asia and handsome French colonial structures. Now, rapacious real-estate companies in league with the ruling Cambodian People's Party (CPP) are demolishing much of that past as they take advantage of rising land prices.

"I don't think the CPP care about or respect architecture. They don't care about the French colonial heritage or my work or that of my contemporaries," says Vann Molyvann, a tall, frail 87-year-old, who nevertheless remains dapper, dressed in an all-grey ensemble of trousers, shirt and tie. "The government could do something to preserve the buildings. They work very closely with the property developers and could tell them. But they're only interested in selling the land."

Vann Molyvann's advanced years have not diminished his passion for architecture or his fondness for speaking his mind. Inside his house, where the air-conditioning units sit flush in the ceilings and natural light floods into large wooden-floored rooms, his desk is covered in old blueprints and scholarly tomes, which he refers to constantly.

"I spend all my time thinking about how I'd manage the planning of Phnom Penh. I don't have anything else to do," he says, with a smile.

Watching the transformation of the city is particularly painful for Vann Molyvann. Not only was he Cambodia's chief town planner in the late 1950s and 60s, he was also the kingdom's state architect, and thus responsible for many of the buildings that are being torn down to make way for the steel and glass skyscrapers.

In a remarkable burst of creativity between 1957 and 1970, Vann Molyvann designed, or helped plan, almost 100 structures. Ranging from the Arc de Triomphe-inspired Independence Monument and the iconic National Sports Complex, still Cambodia's largest venue and once the finest arena in all of Southeast Asia, to banks, breweries and universities, Vann Molyvann's buildings helped turn Phnom Penh into perhaps the most eye-catching city in the region.

Combining the influence of Le Corbusier's austere modernism with the ancient tenets of Cambodian architecture that produced such wonders as the temple complex of Angkor Wat, Vann Molyvann's buildings were far ahead of their time. Both environmentally conscious and utilitarian, they appear avant-garde even now.

"Whenever I show Vann Molyvann's work to foreign architects or organisations like Unesco, they can't believe that this is indigenous architecture created after Cambodia became independent," says Helen Grant Ross, an Anglo-French architect and co-author of Building Cambodia: New Khmer Architecture 1953-70. "He invented something new. His work is much more audacious and innovative than any British architecture of the 1960s."

Yet, for all his achievements and status as the finest modern architect to emerge from Southeast Asia, Vann Molyvann is an unheard prophet in his own country and barely known overseas. That neglect is due mostly to Cambodia's turbulent and tragic recent history. The advent of Pol Pot and the genocidal Khmer Rouge regime forced Vann Molyvann into exile for 20 years and, when he returned, architecture suddenly seemed less important.

"It felt like coming home to a different country, when I returned in 1991," says Vann Molyvann. "There were only 14 members of my family still alive. One of my brothers had been killed by the Khmer Rouge and I had to help my other brothers find work so they could send their kids to school. It was a time for reconstructing families."

The Khmer Rouge, who hated urban existence to the point that they largely de-populated Phnom Penh, made half-hearted attempts to erase Vann Molyvann's work. Having decided to abolish money, they tried but failed to blow up his landmark National Bank of Cambodia building in the southern port town of Sihanoukville, although they used the National Sports Complex for mass rallies. But while most of his buildings survived the Khmer Rouge era, they are now up against a more formidable opponent in Cambodia's strongman prime minister, Hun Sen.

Since 1991, about 10 per cent of Phnom Penh's population have been forcibly evicted from their homes, as domestic and foreign companies buy up whole neighbourhoods and redevelop them in collusion with corrupt CPP politicians. For Vann Molyvann, that is state-sponsored vandalism at best.

"Phnom Penh is now being planned in totally chaotic fashion," he says. "It's not really town-planning, they're just building everywhere. I would leave the centre of town as it is. But how do you stop the South Korean and Chinese investment?"

Some of Vann Molyvann's most impressive buildings are gone forever - in 2008, both the National Theatre and the Council of Ministers building were demolished. The White Building, a vast network of apartments the design of which Vann Molyvann oversaw, has been neglected to the point where it is little more than slum housing. Some of his other buildings, such as the Phnom Penh Centre, have been renovated with little thought and are barely recognisable as Vann Molyvann's work.

Most upsetting for the architect is the fate of the National Sports Complex. Built to host the 1963 Asian Games, the 60,000-seat arena incorporated an intricate series of pools to protect it from monsoon floods, a deliberate imitation of the moats surrounding Angkor Wat, and Vann Molyvann regards it as his crowning achievement.

"To be in charge of such a big project at such a young age was exceptional for me," says Vann Molyvann, who was just 36 when he designed the complex. "It's my favourite building, along with my house. When King [Norodom] Sihanouk opened the stadium, it was a hugely emotional moment for me."

But in 2001, the arena was sold to a Taiwanese company. It filled in the pools and constructed offices and shops on parts of the grounds, completely compromising Vann Molyvann's vision.

"I don't even want to see that building anymore," says Vann Molyvann, anger flashing in his tired but bright eyes. "The changes are nonsense. They reflect a lack of understanding of our culture, of culture itself. The Taiwanese company see it as a profit-making building; they have no concept of art. We created that building out of the traditions of Angkorian architecture. But the government doesn't care about all that tradition."

Many believe it is only a question of time before the stadium is razed.

"The National Sports Complex sits on 30 hectares of land in the middle of Phnom Penh and that's worth a lot of money now," says Grant Ross. "There's a real possibility that a lot of Vann Molyvann's buildings will disappear. If they don't make any money for the developers and the crony politicians, they'll be squeezed into oblivion."

Such a fate was unthinkable when the arena opened and Cambodia was going through its own version of the swinging 60s. King Sihanouk had won Cambodia's peaceful independence from France 10 years earlier, in 1953, and the country was enjoying a cultural revival as neighbours Vietnam and Laos were enduring war.

Until 1970, when King Sihanouk was overthrown in a coup, Cambodia was in its second "golden age" - a deliberate reference back to the time between the ninth and 14th centuries, when the Khmer empire was at its height and Angkor Wat was built. Filmmaking and the arts flourished while a vibrant music scene that, like Vann Molyvann, took its cues from both home and the West emerged as the country underwent an extraordinary cultural rebirth.

"A lot of Khmers who had studied abroad came back and were very keen to explore their culture," says Vann Molyvann. "There were new movements in music, film, dance and art. For example, the Royal Ballet sent people into the forests and mountains to record traditional folk dances and to incorporate them into modern dance. It was a huge renewal of our culture.

"It was a heady time and one I look back on with huge affection."

Vann Molyvann was one of those Khmers who had returned to the country post-independence. Born in 1926 in Kampot province, southwest Cambodia, the son of a minor civil servant, Vann Molyvann won a scholarship to study law at the Sorbonne, in Paris, France.

"No one in Cambodia really knew what architects did, but everyone knew lawyers could find work so I chose to study law," says Vann Molyvann. "But I failed the Latin part of my Roman law exam and so I went to the School of Fine Arts to study architecture instead."

His time in France, and the years he spent in exile in Switzerland with his Swiss wife, has resulted in French being Vann Molyvann's preferred foreign language. And it was the Swiss-French architect and urban planner Le Corbusier who would be his greatest inspiration.

"Le Corbusier was at the forefront of the movement for planned cities," says Vann Molyvann. "But I was also influenced by the garden cities being built in the UK. I liked both concepts."

By the time he returned to Cambodia, in 1956, Vann Molyvann was one of only four foreign-trained architects in the country. A year later, and aged just 30, he was appointed Cambodia's chief architect by King Sihanouk and began work on the Chaktomuk Conference Hall, his first building. Located on the banks of the Tonle Sap River and close to the royal palace, the startling, fan-shaped hall is one of the few Vann Molyvann projects to have been properly maintained, and it remains in regular use.

The king's support proved crucial for Vann Molyvann.

"Sihanouk gave young people the chance to take great responsibility," he says. "I liked him. He didn't act like a king with me. He was very human, very friendly. Once, I had to do a rush job to fit out a boat for him. I worked day and night and was so tired at the end of it, I just collapsed. Sihanouk hadn't heard from me and was so worried he sent the army and a helicopter to look for me. That was the relationship we had."

Even more important than King Sihanouk's friendship was his power and wealth.

"Architects aren't like artists - they can't paint in an attic - they need a benefactor to provide them with funds," says Grant Ross. "Without Sihanouk, Vann Molyvann might not have built anything.

"Sihanouk was a creative person himself and he pushed Vann Molyvann. He knew planned modern cities would stimulate the economy and so he placed a great importance on architecture."

That patronage enabled Vann Molyvann to become a government minister and the first rector of the Royal University of Phnom Penh, as well as to design at a furious pace. But Vann Molyvann's real genius was his ability to adapt the concepts of modern Western architecture to the needs of a developing country.

"Places like Hong Kong and Singapore have glass and steel buildings because they can afford air-conditioning. Vann Molyvann knew Cambodia couldn't afford that, so he got round it by orienting all his buildings north, so the sun isn't shining directly in and they can be kept cool," says Grant Ross. "He raised his buildings off the ground, which is logical for a city like Phnom Penh, which is located in the Mekong river basin and prone to floods."

Despite his huge impact on the planning and look of Cambodia's capital, Vann Molyvann remains little known in his home country.

"I hadn't heard of him until I started studying architecture. But even then, we weren't told much about him," says Keo Rottany, a 33-year-old architect in Phnom Penh. "We didn't study any modern Khmer architecture. We learned only about traditional Angkorian architecture. It was only when I did my master's [degree] in Japan and spoke to foreign architects that I began to understand his achievements."

For many Khmers, Vann Molyvann's buildings are still too different, especially when compared to the more picturesque villas and mansions left behind by the French.

"Architecture isn't a popular subject in Cambodia and a lot of people don't really understand the meaning of Vann Molyvann's work," says Hok Sokol, a 40-year-old architect in Siem Reap. "I think if they did understand it, then there would be more interest in preserving the buildings."

Vann Molyvann's career as a high-profile working architect lasted only 13 years. Along with his wife and six children, he went into exile in 1971, as Cambodia descended into chaos ahead of the Khmer Rouge takeover, in 1975. He spent much of his time abroad working for the United Nations on housing projects for developing countries.

When he returned to Phnom Penh, he found his house had become the government office for land administration. After petitioning for its return, Vann Molyvann devoted the rest of the 1990s to working for an NGO charged with preserving Angkor Wat. It was during that period that he first encountered Hun Sen.

"He allowed Unesco experts to come in and help to save Angkor," says Vann Molyvann.

Now, though, the architect is a firm supporter of the opposition Cambodia National Rescue Party. He believes its members to be more committed to preserving what is left of the Khmer architecture of the 50s and 60s as well as Phnom Penh's remaining colonial buildings.

"The French wanted to leave a legacy behind them, so they sent good architects here who weren't building for profit," he says, although he is not optimistic about the future of his own work. "I don't think my buildings will survive. I despair for them really.

"I want my house to be rented out when I am no longer here so the money can be used to maintain it as a monument and museum. Just like Angkor Wat was saved by Unesco, it will be up to my family to conserve this house, not the government."