The following correction was printed in the Observer's For the record column, Sunday November 5 2006

The Maggie's Centre for Lanarkshire, referred to in the article below, is designed by Reiach & Hall, of Edinburgh.

Zaha Hadid, the most acclaimed female architect of our age, has struggled for years with a hurtful dilemma. Despite her fame and success, despite an international reputation that has seen her win the biggest prizes and the most exciting commissions, and despite making London her professional headquarters for 26 years, she has never built a building in Britain. 'It's ridiculous,' she says in the vast conference room at her office in Farringdon, London. 'I have no idea why they don't choose me! I can't speculate any more. Nobody has actually come up to me and said, "They don't want you here ..."'

In a few days, this peculiar state of affairs will come to an end, when her first building - small, elegant, perfectly designed for its intended use - will be unveiled in the grounds of the Victoria Hospital, Kirkcaldy, Fife. It is a tribute to a friend of hers, and unlike most buildings that go up by stellar architects these days, it has partly been made possible by the old-fashioned notion of public subscription. Hadid has worked on it for expenses only, and it may turn out to be the most life-changing piece of work she has ever been involved with.

'The idea was that it should be hovering over the edge,' she says as she consults her drawings. 'The idea for the building is that it's like a fold, like one whole piece that just wraps around. These shapes here are openings for light ... inside there are curved walls ... here you can have a consultation with a nurse, and here you can have a relaxation course, and here is the toilet, and this is the library area, and here is the main kitchen area with this big table where you can have tea and just chat to whoever else is there.'

The building she has designed, which cost a little over £1m to construct, is quite far from the avant-garde anti-gravitational creations that sealed her reputation. It is, in fact, like a small house, which befits its purpose: a home from home for people with cancer.

Hadid, 55, manages to combine an air of imperious regality with that of a comely matron. She was born in Iraq, but her creative career took flight at the Architectural Association in London. It was here, in the mid-Seventies, that she became friends with Charles Jencks, a teacher at the AA, and Maggie Keswick, one of his students. Jencks, an American, was establishing himself as a leading architectural critic and theorist, while Keswick, born in Scotland, would later establish herself as a writer and designer specialising in landscapes and gardens. They met Hadid primarily through mutual friend Rem Koolhaas.

In 1988, at the age of 47, 10 years after she married Charles, Maggie Keswick Jencks was diagnosed with breast cancer. She had a lumpectomy followed by a mastectomy, and in the next five years did some of her best work; she wasn't the sort of woman to hang around waiting for the worst. Five years later, the cancer reappeared in her bone, bone marrow and liver, and she was given a grim prognosis. A specialist advised her not to compound her problems by searching for desperate treatments or quack remedies, and if it hadn't been for her two children she might have given up. Instead, with the help of her husband, she gathered as much information as she could from all possible sources, occasionally travelling abroad for new therapies.

Her main course of treatment took place at the Western General Hospital in Edinburgh, where she underwent a course of high-dose chemotherapy and stem-cell replacement. She also changed her diet and learnt reflexology, and the more she took control of her treatment, the better she felt. She was surrounded by people doing their best to help her, but she was still often confused by the wide variety of choices presented to her, and her grasp of aesthetics made her wonder if the strip-lit clinical environments she spent so much time in couldn't be improved to alleviate the stress. 'I would drive her for two hours to her chemo,' her husband remembers, 'and then we would often wait for an hour in a room with no windows, and on the way back, when she was often in pain, she would talk about how she didn't mind waiting, but that she wished it could at least be in a nicer place.'

Maggie was encouraged by her surgeon to put some of her thoughts on paper, and the result was an article entitled 'A View from the Front Line', published in the medical journal The Breast. 'I am a sanguine character,' she wrote, 'and for 48 years my life was exceptionally easy. So easy that for me breast cancer seemed almost like a payment of dues.' She went on to describe how most of the treatment and advice she received was well-intentioned and often of a high standard, but it was clear that the NHS found it difficult to cope with the huge pressures placed upon it ('I'm so sorry dear, but could we move you to the corridor?')

She read that having cancer was a bit like a parachute jump behind enemy lines, and she agreed. 'There you are, the future patient, quietly progressing with other passengers towards a distant destination when, astonishingly, a large hole opens in the floor next to you. People in white coats appear, help you into a parachute and - no time to think - out you go.' Her account, which her friends point out is just as quick-witted and eloquent as she was in person, goes on to consider her own case in some detail - how she learnt about Chinese medicine, the part played by meditation in her recuperation, her erudite but relaxed attitude towards dietary supplements and nutrition.

Maggie reached a significant conclusion: what matters most is not to lose the joy of living in the fear of dying. She found that the more information she obtained, the less fearful she became. But she continued to loathe her environments: 'Waiting areas could finish you off.' Her experience was one of tiring expeditions round huge institutions, terrible lighting and miserable seating. She saw how 'patients who arrive relatively hopeful soon start to wilt'. And she felt there was nowhere she could go to cry.

At the time of her writing in 1994, she was in remission, but the cancer returned within a few months. She resolved 'to die as well as possible', and she did so in June 1995. But before she died she laid the groundwork for an important new step in the way this country cares for its cancer patients and those close to them. The idea could be considered revolutionary if it wasn't so straightforward, and so overdue.

Maggie had visited the Wellness Foundation in Santa Monica, and her spirits immediately lifted. Here was a vision for ideal care, a place where the patient was not just another unit being processed through an entrenched hospital system. And so she and her husband and her friends began thinking about a place where waiting time could be used positively by studying the latest literature, or relaxing, or talking to other patients in a comfortable but inexpensive environment. They imagined that this place would have a large kitchen with a kettle seldom off the boil.

It would ideally have a placid and healing view, and do everything to discourage gloom. Soon the architect Richard Murphy drew up plans to convert a stone stable block in the grounds of the Western General Hospital, Edinburgh, into a place of colour and light. Maggie studied the plans with her husband in the days before she died, and discussed her ideas with her oncology nurse Laura Lee. 'Her big thing was that the patient should feel empowered,' Lee says. 'And to feel that your life doesn't end when you get your diagnosis.'

And so it was that the first Maggie's Centre opened 100 yards from the hospital's oncology unit in 1996. Since then, three similar drop-in centres have opened - in Glasgow in 2002 (designed by the local firm Page/Park), in Dundee in 2003 (Frank Gehry), and Inverness in 2005 (Page/Park again). The Maggie's programme also runs within the Churchill Hospital, Oxford, and is due to get its own building there in 2008. Site work has just commenced on the Maggie's Centre designed by Richard Rogers on the edge of the Charing Cross Hospital, and there are more planned in Cambridge, Cheltenham, Nottingham, Swansea and Lanarkshire, by the architects Daniel Liebeskind, Richard McCormac, Piers Gough and Kisho Kurokawa.

Zaha Hadid's building is not the sort of thing you would expect to see within the grounds of any NHS trust hospital, much less one whose main exposed feature beyond its rectangular tower block is its car park. But Hadid and her client spotted an opportunity. On the edge of the car park there is a wilderness dipping into a hollow, and by placing a small building on its edge facing away from the hospital a sense of calm may be obtained by those within it. The seamless fold of the steel structure suggests a single cell before it starts to multiply and cause havoc; after the bad news and the uncomfortable treatment in the main building, a person may return to a state of wellbeing and poise in an atmosphere of mutual support.

The outside is clad in a black liquid-applied polyurethane coating with a silicon carbide grit that makes it glisten in the sun. Inside, the scene is white, the curves and ramps contrasting with the taut external frame. Once again, allegory: the harsh reality softened by clean comfort and impregnability.

'I think that fundamentally architecture is really about wellbeing,' Hadid told me. 'Every building you make, people should feel good in it.' Hadid's most famous works - the Vitra Fire Station in Weil am Rhein, the Tram Station in Strasbourg, the Contemporary Arts Center in Cincinnati, the Phaeno Science Centre in Wolfsburg - all reflect the tone of their intended purpose. Her Maggie's Centre in Fife is no exception. She remembers conversations with Maggie Keswick Jencks in hospital about the impact of surroundings on one's mental health, but only began to fully interpret the brief for her project after visiting Scotland's other Maggie's Centres and imagining an interface between the isolation and turmoil that treatment for cancer can create, and the ability to alleviate this with the benefits of shared experience, professional guidance and fortitude. In strict architectural parlance, Hadid concluded that she was actually building 'a place were people can just chill out'.

I was escorted inside the structure earlier this month by the project manager Tiago Correia, and we had trouble hearing ourselves above the drilling and the slapping of paint. The gas and electricity had recently gone in, and the shelves were going up. The furniture had just arrived from Germany. Once in a while, Correia broke off from the tour to observe how the light affected the outside walls: 'Look, there's the sparkle!' He was born in Portugal, where he had seen cancer strike his grandfather. He showed me how the hard furniture followed the curves, and the computer shelf and seat formed part of an inner wall. Then there was the library area, the counselling room, the staff office and toilet, and the clever curtain-and-door system that could either open up all the rooms to the hub of the kitchen, or seclude them. He was fairly sure everything would be ready and smoothly coated by the time Gordon and Sarah Brown come up to open it at the beginning of November.

The building will have three full-time staff and two part-time benefit advisors, and the range of help on offer will include sessions with clinical psychologists, art therapists, nutritionists and beauticians; there will be assistance with prostheses, stress management and much advice for family and friends. One of the permanent staff will be Elizabeth Adams, who for the past two years has steered the fundraising campaign. The centre received £400,000 from the Lottery, and has raised more than that again from a number of private donations and the usual dinners and sponsored hikes to exotic lands. 'It's captured the imagination of Fife,' Adams says, 'for who hasn't been touched by cancer?' The fundraising continues: with all services provided free of charge, the annual running costs are estimated at between £250,000 and £300,000.

Maggie Keswick Jencks's oncology nurse is now the chief executive of the Maggie's empire. A decade after the first opening, Laura Lee says, 'The centres have become a beacon, but it still feels that we're not going fast enough.' Lee finds that everywhere she goes in Kirkcaldy she hears people talking about the new building. Not everyone is quite sure about the shape of it yet - the public have not been inside, after all. But Lee says that Hadid has got it 'spot-on', and she feels her own anxieties drop away as she walks in. She hopes that its users will soon feel 'hugged by the building', the way they do in the others.

'Maggie would have been amazed, delighted, stupefied,' Charles Jencks says. 'She did the blueprint, but would never have imagined the impact.' Jencks uses a good word to describe the ethos of his former wife's concept, and its subsequent interpretation by passionate architects: kitchenism. 'You can just go in and slip into a relationship without having to fill in forms,' he says. 'It's an institution which is not an institution, a home that is not a house, a church which is existentialist.' And he may as well have added: a door that is always open.

· To make a donation to the running costs, please go to www.maggiescentres.net