To be Frank

This is because of Gehry's relationship with the client, Maggie Keswick Jencks, who founded Maggie's Centre, a charity that helps people with terminal cancer. "This is the one project he has done in his life that he has lost sleep over," says Fred Stephen, partner at James F Stephen, the local executive architect for Gehry. "Maggie was very important to him, he also said it was his favourite building." Gehry's practice has used the advanced modelling techniques it usually reserves for its larger projects, resulting in a building as radically shaped as his other work, like the Guggenheim Museum in Bilbao, Spain. The local team has the heavy responsibility of realising Gehry's vision down to the last detail.

The building occupies a small plot at the edge of the Ninewells Hospital site in Dundee, Scotland. Today it is hard to make out the normally impressive view over the Firth of Tay to the hills beyond because of heavy rain. The weather has driven even the local builders home, so the site is quiet apart from the drip, drip of water falling from the building's roof to the saturated ground. Fortunately, most of the roof is finished and clad in its stainless-steel rainscreen shingles. Its sculptural curves echo the local landscape – a mass of hills and valleys blending into one another. One particularly sharp peak stands out against the others as it is still waiting for its protective coating of stainless steel. Adjacent to the roof, a small two-storey tower, made from composite timber board, is obscured by scaffolding. This will eventually be rendered and painted white.

The building is tiny and has just one storey, apart from the tower opposite the entrance. The single-storey section will contain a central circulation area next to the main entrance. To the left will be a large kitchen and a common room, with small consulting rooms to the right. The tower will house a library on the ground floor and a private family room above. The south elevation will be extensively glazed to make the most of views over the Tay estuary.

This is the third Maggie's Centre to be built. Maggie met Gehry through her husband, architectural critic Charles Jencks, and they became good friends. They also worked together on garden designs for one of Gehry's projects. Sadly, she died from cancer in 1996 at the age of 55, after setting up the charity. Before her death she asked Laura Lee, her chemotherapy nurse, to carry on her work.

Lee has taken up this role with extraordinary energy, and several more Maggie's Centres are in the pipeline. She and Jencks had thought of asking Gehry to design the Maggie's Centre, Dundee: "The idea of Frank came up, but we thought he wouldn't be interested in such a small building. We asked him and he immediately said yes."

The brief for the building is very simple. Lee explains: "We wanted to create a building for people with cancer that was a home, yet not a 'home', that was uplifting to the spirit; you walk in and see a kitchen, trees and the Tay estuary. We wanted somewhere that could communicate with people, and a place where people could also get practical help and advice."

Gehry started work on the centre in 1999. According to Jim Glymph, a senior partner at Frank O Gehry & Associates, a number of factors influenced the design. "He doesn't start with any preconceptions," says Glymph. "Factors influencing the design included Frank's feelings for Maggie, his conversations with the client, and the site, which was special." Glymph also says Gehry was influenced by the folds of fabric in a Vermeer painting, and that he was interested in how metal reflects light. "All these things get stirred around and the design evolves."

Gehry has used technology he normally employs on much larger projects to develop the design. Famously computer-phobic, Gehry works up designs using real models. These are "digitised" and fed into a computer as a 3D virtual model. From here the design is progressively refined, with more physical models made and changes fed into the computer – a process that is repeated many times.

No less than 23 models were made as the design of the building evolved; Gehry sets great store on the physical models. "The computer model works in parallel with the physical model, but is always one step behind," explains Glymph. "Frank and most of us in the practice believe the visualisation tools provided by the computer don't let you see the physical design."

A local architect was needed to inform the design process and execute the project, and Maggie's Centre selected James F Stephen. Project architect Mark O'Connor explains how he felt when the practice was selected: "We were very aware of the building – it was tremendously exciting for the UK, then for Scotland, then Dundee – then it arrived in our office." He continues: "Our job was to replicate the design full scale. We had to get through planning, do the drawings and the specification, and get as close to Gehry's final design as we could."

The practice had to learn to work the Gehry way, including working with Gehry's computer model. Gehry uses a computer program called Rhinoceros, a 3D modelling program associated with product design. "We went through a vertical learning curve," says O'Connor. "We had to buy in the Rhino package, sit down and spend two months learning that. The entire building was a learning and research process, as we also had to research the materials.

We couldn't take anything for granted."

The local architect's job was to turn Gehry's vision into reality. Initially this involved checking that Gehry's design conformed to Building Regulations, implementing local construction techniques and detailing – not least of which was keeping the damp Scottish climate at bay. "Everything is detailed for water," says O'Connor. "The issue isn't rain but damp. When it rains the water can lie here for a whole winter. There are lots of nooks and crannies for damp – this all had to be detailed." The team also faced tough height restrictions because there is an emergency helicopter landing pad right next door to the centre.

Once all the information had been fed into Gehry's model, it was signed off to James F Stephen. The local architect then worked up the production drawings from the model for use by the contractor and specialists on site. Each one had to be approved by Gehry's office, which could have been a difficult process, but O'Connor and Stephen insist that they found working with the Gehry team enjoyable. "Gehry's a lovely man, a man of great humility who is also fun to be with," says Stephen. "The Gehry people were very helpful – they are grounded, not prima donnas," says O'Connor. "They are very positive and inspirational. I remember being bowled over by how keen they were. They said: 'What would you build, given no limitations?' – then they said: 'Let's remove the limitations'."

When the process of turning the design into working drawings was complete, James F Stephen made a detailed physical model to check the design could actually be built on site. The next challenge was to find specialists up to the task of building it. Initially, Stephen thought of using boatbuilders to make the roof structure, which comprises a net of beams bent in two planes, and twisted in places. Stephen says this would have been expensive but seemed the only option at the time. However, this approach wasn't met with much enthusiasm: "I phoned one in Arbroath – he thought I was a bit daft."

It was a passing salesman from timber products manufacturer FinnForest, who saved the day. The firm makes a laminated beam called Kerto. "The Kerto man came out of the blue and was selling it as a straight beam," says Stephen. "We thought, 'Wait a minute, maybe this is the answer'." The "Kerto man" also suggested that the architect approach Cowley Structural Timberwork, a specialist in making unusual, curved timber structures, to build the roof.

Having finalised the roof structure, Stephen had to find a specialist in metal cladding. He was familiar with shopping centre the Forge in Glasgow, which is clad in stainless steel panels. So he found the name of the company, WB Watson, and asked them to do the roof. Other contractors, such as the bricklayer, are local to Dundee. All the specialists are contracted directly to the client rather than the contractor.

Because the project was so complex, HBG was employed to oversee it – at cost. Coyne, who managed the project for HBG, is a trained civil engineer with the skills necessary to deal with the difficult geometry. Every single curve in the building is different, with very few curves following one simple radius, which means that the building has had to be mapped out as a series of points. "The problem is what happens between the points," says Coyne. "You can take a very accurate detail and move it slightly to the left or right and it's different – there is a degree of interpretation between the points." Coyne still has to follow the drawings as carefully as possible, though. "If you change one aspect of the geometry it affects the rest of the structure," he explains.

The specialist contractors also found the building mind-boggling. "The interface between a complex body of digital information and how people deal with this on site is a major challenge," says Coyne. "You had local contractors at the start scratching their heads and saying 'we can't do that'." He says about half of the people who turned up to work on the project quietly disappeared as they realised they simply were not up to the job.

The building sits on conventional strip foundations that in turn support brick-and-block walls capped by a reinforced concrete floor slab. The next job was to set out a whole series of steel posts to support the roof structure – the posts had U-shaped heads to engage with the primary roof beams. It took three days to set out the posts, and one day to erect them. The setting out was done using the same co-ordinates system as for determining a location on an Ordnance Survey map, using the helicopter landing pad as a reference point. When these were all in place, Coyne was presented with a confusing scene. "I couldn't deduce the shape of the roof. I just accepted that was where it had to go," he says.

The next stage was to put up the primary roof beams made from Kerto. The spaces between the beams were infilled with rafters on site. The roof is given stability against lateral forces by a central toilet block, and by the curved brick walls, which will be rendered.

Coyne found building the tower particularly difficult. "It's shaped like a Piat d'Or bottle," he comments. It tucks in at the base then bulges out in the middle and in again at the top. Furthermore, it is not circular in cross-section, but an ellipse.

The tower is made up of 16 pre-manufactured segments bolted together. Getting each panel to line up correctly so the whole tower is exactly the right shape took a week longer than planned.

Coyne describes the setting-out process as "tortuous" because they had to establish the correct setting-out point from scaffolding, often in the midst of a gale. Coyne says it was virtually impossible to determine whether it was the inaccuracies in setting out the points or the weight of the panels that affected the shape of the tower. The only way to get it right was loosening the bolts holding the panels together, jiggling them around so they lined up, then retightening the bolts.

The main roof will be finished in two weeks, "before the winter really sets in," says Coyne. The next stage is to render the tower and the walls, then install the windows. After that, work can start in earnest on the interior. The building will be finished in April next year and will be a unique contribution to Dundee's skyline, and a splendid tribute to both Gehry and Maggie Keswick Jencks.