At the National Gallery Singapore, deception - of a beautiful kind - is at work in its spaces.

The modern art museum is converted from the former Supreme Court and City Hall buildings, which were built in different decades in the early 1900s and designed by different architects.

Inside the barely year-old museum, however, one hardly notices the differences, or finds it strange that the historic buildings are both old and new at the same time. They feel one, organic and current.

This is what the museum's chief architect, Mr Jean Francois Milou, 63, had intended it to be. He pulls off this sleight with an element that is invisible, but by which everything is seen - light; specifically, a soft, golden light.

The thoughtful, sensitive founder of the studioMilou architecture firm says: "The light is a common element everywhere, so we tried to unify the buildings with this even, soft and warm light, which is not the light you have usually in Singapore. It's a creation of the project."

This soft, golden light is most evident when one stands in the former Supreme Court courtyard with sunlight streaming in through the pale gold filigree metal roof.

The roof is a key feature of the museum's design, a dignified way of signing the buildings' new identity while connecting them to each other.

The idea for it came to Mr Milou in 25 minutes, on a hot day in Singapore in 2007. That half-hour would prove to be a turning point in his life, binding his fate, fame and future to this land.

The French-born architect had been keen for his studio in Paris, which is experienced in the adaptive reuse of heritage buildings and sites, to take on work beyond Europe. It was at that time that he learnt about the design competition for the Singapore museum.

As a long-time admirer of Singapore's urban planning and having passed through the country a few times, he warmed to the idea of the competition.

On that day in 2007, he was sitting in the Padang on a plastic chair borrowed from the Singapore Cricket Club tennis court, facing the former Supreme Court and City Hall buildings with the sun bearing down on him.

"I was trying to meditate on the best geometrical way to connect the two buildings," he says. "I had the idea that it should be just a line, a way of signing the change, but in a very discreet and elegant way. But how do you make that graphic idea architectural?"

He imagined a piece of material floating over the two buildings, found an envelope in his pocket and began folding it to find a structural solution.

The result was a veil-like canopy that links the two buildings at roof- level and drapes down the sides to envelope the space separating them.

"I try always to have a vision of what I will develop. But it's a very rare moment, in fact, to have a vision of something that doesn't exist," he says deliberately, as if savouring that moment again during the interview with The Straits Times at his office in The Adelphi.

Happy with the design, he submitted it for the competition, but harboured no hopes of winning. "You are one of a few hundred applicants and there is only one winner," he says candidly.

Yet his design went on to be among the three shortlisted from the 111 entries and was eventually picked the winner in 2008 by an international jury panel chaired by Professor Tommy Koh, Ambassador-at-Large at Singapore's Ministry of Foreign Affairs.

Mr Milou's firm then partnered home-grown design and engineering company CPG Consultants to convert the national monuments into the museum.

Coming up with the design, Mr Milou says, was 5 per cent of the work. The remaining 95 per cent was seven years of intense labour that saw him and his team of 20 architects in Singapore spend day and night overseeing the complex, large-scale architectural project from start to finish.

He says: "This is not a project where you make a design and send it off to be fabricated by a contractor. This is like working on an archaeological site. You have to adjust the design to the finds of the archaeology, which surface daily, more or less."

To meet the demands of the project, he opened a Singapore branch of his practice in 2008 and relocated his family from Paris.

His two younger daughters, Clara, six, and Anna, four, were born in Singapore, and his eldest daughter, Elsa, 10, is enrolled in a Singapore primary school. The family is staying in a serviced apartment while they search for a place of their own.

He also became a Singapore permanent resident - one with a penchant for durian, teh "with a lot of condensed milk" and soft-boiled eggs "with a lot of sweet soya sauce and pepper".

Of his decision to take up permanent residency, he says simply: "It's like when you're in love, you're happy to be married."

His team of architects in Singapore is made up mostly of young, home-grown talent, whom he hired for their ideals about architecture.

He says: "Intuitively and naively, I was not looking so much at the experience of the people, but their ability to focus on producing something beautiful, something touching. Rather than being surrounded by people interested in pouring concrete, I'm more interested in the kind of emotion they can create in the mind of the visitor when they create a space."

The work of conjuring a moving emotional landscape in a museum visitor through the experience of the built environment, however, is tiring, tedious work.

A major challenge was to incorporate all the modern needs of a world-class museum, including high-tech lighting, climate control and security features, in the historic structures without changing the buildings' integrity and character.

To achieve this, the network of technical services had to be rendered invisible. This meant concealing them in the ceiling, yet without altering the elaborate timber-panelled ceiling in the courtrooms.

His attention to detail, staff say, is evident in his habit of asking if something has "all the necessary beauty" - is perfectly done and exactly as it should be - before work on it proceeds.

His wife, Ms Suzanne Ogge, 45, who is head of international development at studioMilou in Singapore, says: "Jean Francois has a strong vision, but he also expects the best of the staff and because of that, people rise to the occasion."

She adds: "If people make mistakes, he never growls at them, he's not interested in that. He is interested in switching courses and moving on."

His pursuit of beauty, Mr Milou says, is influenced by his mother, who gave up being a doctor after she had him, the eldest of three children. Her eye for fashion and tailoring and love for the arts rubbed off on him while he was growing up. When it came time for him to choose a course of specialised study, he was torn between fashion and architecture.

His father, a professor of philosophy, suggested he try architecture.

"He said, 'If it's not what you want to do, you will know it very soon and you will change, and it will not be a big deal.' I think it was good advice," says Mr Milou, who realised he had a knack for architecture and went on to graduate from the Ecole Nationale Superieure des Beaux- arts of Paris.

His interest in the adaptive use of heritage buildings and sites developed organically after the firm won a competition in 1991 to build a museum around the remains of a mediaeval chapel near a Neolithic burial site in Bougon, France.

The successful project drew media attention and other similar projects came his way. He has also worked as a consultant for the Unesco World Heritage Centre and the French government on projects in India, Nepal and Indonesia. The National Gallery Singapore is his first major project in Asia.

The museum's chief executive Chong Siak Ching, 58, says: "Jean Francois Milou had skilfully and sensitively transformed two beautiful historic monuments into a stunning National Gallery without losing the integrity of the original architecture; in fact, enhancing it.

"His eye for preservation and innovation breathed new life into the gallery's buildings, allowing visitors to enjoy art, architecture and history all at the same time."

The warm reception to the museum - it won both the President's Design Award and the URA Architectural Heritage Award last year - has moved Mr Milou to make Singapore his home base.

He says: "We have an office in Paris, but we want to focus more on the office here for the future and develop the work we do in Singapore and the Asia-Pacific."

The firm's other projects in the region include conference centres in Vietnam as well as residences here.

The National Gallery Singapore, however, has a special place in his heart.

He says: "I've made a lot of museums before. This was the most difficult, but in a way, the most rewarding because of the recognition from the people. This is also the most beautiful museum I've ever made, so I'm very proud of it."