Maggie Keswick Jencks was a designer with a passion for gardens. As she was dying of cancer, she created the blueprint for cancer care centres that recognise how design can help recovery. Here friends and family recall a remarkable woman

When Maggie Keswick Jencks was 47 she was diagnosed with breast cancer. Five years later she started to have severe back pain and, after two misdiagnoses, went to her local GP's surgery in Dumfries where she was told the cancer had spread to her bones, liver and bone marrow. In a home video, made for her mother, she described what happened next. She and her husband were told to see a visiting Edinburgh consultant. They waited in an "awful interior space" with neon lighting and then the nurse told them to come in. They asked: "How long have we got?" To which the doctor said: "Do you really want to know?" "Yes we really want to know."

"Two to three months."

"Oh…!"

And then the nurse explained. "I'm very sorry, dear, but we'll have to move you out into the corridor, we have so many people waiting." They sat in a "windowless corridor trying to deal with this business, having two to three months to live. And as we sat there, various nurses who I knew came up and said, very cheerfully, 'Hello dear, how are you?' 'Well,' managing a laugh, 'I'm fine!'"

This was the story that became Maggie's spur – the NHS corridor that would lead to her big idea. There might be no cure for Maggie's cancer but here was something that could be changed. Why shunt people with cancer into miserable surroundings? Didn't people need respect, time and space? With the support of her young nurse, Laura Lee, Maggie would devote the rest of her life to planning a cancer caring centre. She had a feel for what was needed and the drive and money, as daughter of the director of the Scottish trading company Jardine Matheson, to do something about it. She understood the need to feel in charge (not a helpless passenger in a hospital production line). She realised people might want to find out more about their treatment options. And she knew a beautiful space was needed in which to digest even the worst of news. She envisaged a room with a view – and a library. And she argued for an "old-fashioned ladies' room – not a partitioned toilet in a row". This would supply "privacy for crying, water for washing the face, and a mirror for getting ready to deal with the world outside again". She knew that, in a crisis, everything counts, even – or especially – the little things.

"Little" does not describe what has happened in the 15 years since her death: her idea has taken off. Today, there are 15 centres – seven up and running, seven in the pipeline (opening before 2012) and one online (a V&A display this month celebrates the achievement). Yet Maggie's centres are anything but pushy: they are only ever built at the invitation of NHS Trusts and, usually, in the grounds of hospitals with oncology departments. Unsurprisingly, hospitals recognise they need them and architects are queuing up to build them. And the existing centres have tremendous architectural prestige (Frank Gehry, Zaha Hadid, Page and Park have all designed one). But they are about far more than architecture for architecture's sake. Above all, they remind us that it is not frivolous to care about design. And this year the British Medical Association is also, at last, acknowledging architecture's importance as an ingredient in recovery. It is calling on healthcare organisations to "prioritise design in future building projects" after a new report showed that "architectural environment can significantly affect patients' recovery times." We are – to some extent – what we see.

I visited London's Maggie's centre for the first time last summer. Designed by architects Richard Rogers and Ivan Harbour of Rogers Stirk Harbour, it is audaciously beautiful. When you approach it from Fulham Palace Road, you notice it has turned its back on the ugly facade of Charing Cross hospital. It is painted orange in defiance of London's greyness and bad news in all its forms. And its extraordinary roof looks as though it has levitated. The whole building is serenely irrepressible. What is so winning is that it feels like home (although more elegant than any I know) and yet is an in-between space. It is not a retreat or a hospice or a clinic – it is a drop-in centre and it is free. It offers information, advice on nutrition, relaxation classes and a psychologist for anyone needing to talk about the most intractable subjects (depression, fear of dying, dread of cancer returning and other issues not easy to address in a hospital environment).

On the day I visited, the building was full of sunlight. I loved its attention to detail: the bowl of fruit on the kitchen table, the sunhats on pegs in the hall, the lack of signs bossily telling you where to go. And this is the key: you decide what to make of a Maggie's centre. You can walk in and out unnoticed if that is what you prefer. It is perfect for company – or contemplation. I visited at a time when one of my friends was dying of cancer in another country and kept thinking what it would mean to her to have such a place on her doorstep. While I was there, I also found myself thinking about Maggie, as anyone with even a scrap of curiosity visiting her centres must. I wanted to know more about her.

Photographs show a slender, vivid woman with dark curly hair and a dreamer's face when not smiling to camera. One can see the pluck in Maggie, especially in a picture taken at a Scottish picnic, just after her diagnosis – a blaze of a smile on her face, no shadow in sight. I had four people in mind to help me bring her into view: her husband, the writer and landscape architect Charles Jencks, her former nurse, Laura Lee, one of her many friends, Anne Chisholm, and her daughter, Lily. But what I could not predict was that meeting Lily would turn out to be as close to encountering Maggie as could be imagined. Friends exclaim at the likeness between them. And this pleases Lily. She is also following in both her parents' footsteps in having taken up landscape architecture as a career. She designed the garden for Frank Gehry's Hong Kong Maggie's centre and is now working, with Rotterdam architect Rem Koolhaas on the garden for Glasgow's second centre.

Whenever she pictures her mother, she recalls her wearing a green velvet floppy hat "like one of Jamiroquai's". She has the hat but never wears it. "My friends try it on all the time." She ought, she says, laughing, to share it with her elder brother, John (who runs a film production company), as it is "iconic". When I ask about the last two years of her mother's life, she remembers how hard it was to talk about cancer. In some ways, it still is. Lily is 30 now but was 13 then – a "nightmarish" age: "You can't hide anything from a 13-year-old." No one told her what had been said in that Dumfries surgery. But she knew. "Yet, as you know, my mother went on to live for another two years."

She is gentle and unjudgmental about her mother and herself. She understands how hard it was for Maggie to know what to say. Lily remembers her coming into her bedroom to show her hair falling out in clumps. She tried to be honest. She wanted Lily to understand. Yet the need for reassurance was mutual. Lily remembers a car journey to family friends during which Maggie asked her daughter how she looked. Lily said "healthy and full of life". It was only when she heard her mother repeating this verdict, with obvious pleasure, that "I realised how sick she actually was". And there was no Maggie's centre to help them navigate (let alone what is now being piloted in Maggie's Dundee – a support programme specifically directed at teenagers).

As a mother, Maggie was "sensitive" and "careful to spend special time with us alone and was always trying to be aware of our feelings – in case that space got lost in everyday life". Lily remembers cycling with her mother at a time when the effort to be on a bike must have been great but she gave no sign of it. She also remembers how Maggie's bed was often strewn with papers when she was very sick. It is only now she realises they were her precious blueprints for the first centre. "She was very concerned about them," Lily recalls.

It is a tribute to Maggie that her family, her nurse and many friends (some of them architects) have all, after her death, become involved in the centres, as if to keep faith with her. Laura Lee was a young nurse working at the breast chemotherapy suite at the Western General in Edinburgh when they met. She is now CEO of the centres. She is warm and engaging. It is easy to see why Maggie loved her. She does her job impressively too. "The landscape of cancer is changing," she explains, "because more people are surviving. Those with a recurrence live longer. The need for these centres has never been greater because people are living with cancer as a part of life." Laura explains that the centres are funded by investors and public fundraising and tells me about the many and inventive initiatives (such as an annual, sponsored 20-mile night hike through London). Everyone says that Lee is a dynamo without whom the centres could not flourish as they do. Laura remembers the moment at which her friendship with Maggie became "professional" and a shared mission: they travelled the UK and USA researching cancer care centres.

Looking back, Laura had no idea of the scale of what they were starting. Her first impression of Maggie and Charles as a couple was that they had tremendous presence. She found Maggie gentle, polite and "not at all passive". She wanted to know whether she should "go with the sense that her body was deteriorating and weak". She took an intense interest in her treatment. Understandably, her first reaction after the grim Dumfries prognosis had been to give up. But while she was retreating, her husband was in fast-forward, combing the world for cures. In her gallant essay "A view from the front line" (required reading for anyone interested in Maggie or in arming themselves against cancer), she describes the effort of will it took to fight on, and why it was hard: "deciding to give up the certainty of death for the uncertain prospect of a stay of execution: if I got into the fighting mode, and it failed, would I ever get back to this precariously balanced acceptance?" But there came a moment for Maggie and Charles when they, simultaneously, realised they would do everything they could to prolong her life. And Maggie wanted to win more time for her young children's sake as well as for her own.

The breakthrough was discovering that Dr Robert Leonard, at the Western General, was conducting a trial in advanced metastatic breast cancer (high-dose chemo and stem-cell replacement), for which Maggie was suitable. This – and her whole-hearted and intelligent involvement in diet and complementary treatment alongside it – would win her almost two more years. Maggie tended to be positive. She once wrote that the goal for people in her situation – not easy to achieve – was to try not to "lose the joy of living in the fear of dying" – words that have become a catchphrase at her centres. Laura thinks that her focus was always on the luck she had in life. When first diagnosed with breast cancer, she saw the cancer almost as a paying of dues for her privileged life.

Maggie was born in 1941. Her background was not only privileged, it was exotic – divided between Scotland, Shanghai and Hong Kong. Her father was a remarkable tycoon who, during the communist takeover of Shanghai, unlike most Europeans stayed on to help feed a starving population. He was rich but also philanthropic. And he spoke fluent Chinese. Maggie was an adored only child and might easily have been spoilt. But she never was. She was raised a Catholic (becoming, according to her husband, more of a Buddhist/Catholic later in life). She was educated at Woldingham in Surrey and at Lady Margaret Hall, Oxford, where she read English. Anne Chisholm met her at university and remembers her as "light on her feet, buoyant, vividly alive. While the rest of us were creeping around in various stages of lumpishness, here was this dazzling creature – much more stylish than any of us, she could lift a room." Maggie would cook Chinese delicacies in the pantry at her college. The oriental drifted into her dress sense too. She would train a Chinese scarf across her chin, securing it with a knot (in the rum fashion of the time). She was clever, funny, a good listener and a performer (acting in student productions with Esther Rantzen as co-thespian). She sang well and could quote yards of poetry by heart. She was an attractive mixture: an empathetic extrovert.

In 1965, after leaving university, she and a friend launched a boutique, Annacat (named after their dogs). The look, according to Chisholm, was "daughter of Mary Quant". Jencks remembers it more as "Victorian freestyle" (like some outlandish swimming stroke). At that time, Maggie became "a celebrity with the smallest of cs" (in a photograph by David Bailey, she looks the embodiment of the Swinging 60s). But fashion would prove too insubstantial for her. In 1970, she joined the Architectural Association and met Charles Jencks, an American architectural writer and landscape architect teaching there. He remembers how vivacious she was. But he adds: "The thing about Maggie is that she was vulnerable." She was insecure about her looks, did not believe in the beauty others saw in her. She'd spend ages trying to get her clothes right – and be late for dinner. She tended to be late generally. But people warmed to her because as her husband says: 'More than almost anyone I've met, she had a liking for people and they felt that in her."

In 1978, the year in which she married, she published a scholarly book on Chinese gardens. Scotland was always important to her – but China was to be the defining influence. Jencks remembers she made several solo research trips to China, once bringing back for him memorably inconvenient souvenirs: Chinese bullet-hole rocks (called "Scholars Rocks") and "according to ancient custom" a pet cricket in a tiny wooden box which she kept in her blouse and which, during its brief life, drove him crazy. Charles and Maggie understood each other well. Anne Chisholm says Maggie "adored Charlie and his ideas". And she acknowledged that she would not have produced her book without his help. She described herself as a "creative ditherer" – a perfectionist. As she wrote in her acknowledgements, "without his constant badgering and insistence, I should still be on Chapter One." Chinese gardens are seldom written about (Japanese gardens tend to steal the show) but Maggie put these gardens firmly on the map and lectured about them around the world. She described Chinese gardens as "cosmic diagrams, revealing a profound and ancient view of the world, and of man's place in it".

And, strange as it might seem, the passion for Chinese gardens has influenced the Maggie's centres. The sense of the cosmic diagram, the belief that architecture – and gardens – have meaning is essential to understanding Maggie, her husband and the way the centres have evolved. Charles Jencks is a man of tremendous charm and playful erudition (he has dubbed the game of looking round the house he shared with Maggie in London's Notting Hill as "hunt the symbol".) He describes the home, which has become a postmodern landmark, as cosmic. And it is extraordinary to visit it because nothing in it is idly itself. The stairs are an "abstract realisation of the solar year". Maggie's kitchen represents Indian summer. Even the loo has its story. It is wonderful in a way. But it must be exhaustingly inexhaustible to live in it. Apparently, Maggie once said: "I understand, Charles, everything has to symbolise something but symbolism stops at my door." He followed her prohibition to the letter – literally. We inspect Maggie's door – with a carved letter M and open book. And then he shows me her desk which, to his delight, shows her breaking her own rule. She painted it in Johnston tartan: greens and blues with a yellow stripe – as symbolic as could be.

Maggie's centres have their "meaning" too – but of a more instantly graspable sort. "They are," Jencks says, "to do with the way living and dying are part of one thing." There is, he goes on, nothing new about the overlap between health and culture. He cites Hospices de Beaunes and Stonehenge (now thought by some to have been a healing centre) in the sweep of his argument. And in his delightful new book The Architecture of Hope, you can see that each Maggie's centre is different – it is up to visitors to settle on individual meanings. Maggie would have loved unravelling the thinking behind each centre: Frank Gehry's homely centre in Dundee looks as if a child made its roof out of folded foil (you want to pat it); Zaha Hadid's in Fife, with its shark-like exterior made of sparkling silicone carbide grit, allows its visitors to move, in a boldly metaphorical way, from darkness to light; Page and Park's ingenious Inverness centre has a green copper roof and a design based on the idea of a dividing cell. And six new centres are planned to be built before 2012 – Wilkinson Eyre's tree house, which goes on site in Oxford next year, is my favourite: leafily escapist.

But the place that is perhaps Maggie's most personal memorial is not one of her centres at all. It is Portrack, the 18th-century country house in Dumfriesshire she inherited from her parents where, with Charles, she designed a garden now dedicated to her memory. It is a most awe-inspiring place with 60ft manmade mounds and vast lakes, a dramatic discourse between water and land and a swirling exchange of shapes (the lakes were Maggie's design). And it is this place that keeps coming up in conversation. Anne recalls seeing Maggie at Portrack, in her last year and watching her "running up one of the mounds as if she was 20 years old, with nothing wrong with her". Lily can still picture her mother, out in the garden, sketching. And Laura remembers sitting with Maggie at the top of one of the hills, with the sun on their faces. They didn't talk about death, she says, they did not need to, it was understood.

Maggie died on 8 July, 1995. "I remember running away from the hospital," Lily says. "I couldn't believe birds were still singing, the world turning. I remember watching, in disbelief, as someone crossed the road as if nothing had happened." Every morning, she would wake and, for a second, not know her mother had gone – and then the news would hit her again. Even now, she gets upset about it – although she never can predict when she will be ambushed. She would like to be able to ask her mother's advice about the big decisions in her life. She minds that, when she has children herself, they will not have a grandmother. She remembers now how tiring grief was. And, although that has lifted: "I don't think the pain goes away, you just get used to it. There is a hole inside of me but I know it is part of me."

It is winter when I return to the London centre. I walk through Dan Pearson's garden which is planted with 100 birches and runs parallel to one of London's most polluted roads – it seems an act of faith in itself. Some shy hellebores are flowering and Hannah Bennett's smooth sculptures stand out like polished melons – not stepping but sitting stones. Inside, there is a fire in a wood-burning stove and waiting logs, stacked neatly behind glass, are a heartening sight. It is marvellously peaceful. Lily tells me she is often asked, by visitors at the centres, about her mother. "People are really happy to meet me which is so touching. They want to know what Maggie was like." And oddly enough, one of the visitors, while I am there, looks up from the computer – she must have heard me talking – and asks whether I knew Maggie. I hesitate. I am tempted to say: "Yes."