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Few people have been involved in the Terminal 5 project as long as Mike Davies. The Richard Rogers Partnership director started on the job at its inception in 1989. By the time his designs are finally realised at the grand opening of the terminal in 2008, Davies will have been on the project for almost two decades.

"I've worked for five different BAA regimes, or groups of leaders, over the past 15 years – each with a different approach to the problem," says Davies. So, the regime that dealt with the initial concept was succeeded by the regime that took T5 through the longest public inquiry in British history in the second half of the 1990s. It is now under the control of Tony Douglas' team – the one with the job of actually building the beast. And as regimes have come and gone, so the design has altered.

Initially, Davies and his fellow directors at RRP envisaged a terminal that was effectively divided into three parts. It would have three curves on the transparent roof, with a "canal of light" separating the floorplates between arrival on the landside, and the passenger-processing and waiting areas on the airside. These floorplates were to be linked by bridges.

The design was praised in the public inquiry, but in the decade or so that separated initial concept from planning permission, the airport world moved on. Executive lounges became larger, retail on the airside became more important than retail on the landside, baggage-handling changed dramatically and even before 9/11, security checks and security areas had became more substantial, "There was a cry for greater flexibility," says Davies. "The conclusion was that one very large floorplate offered this, so we created one humungous plate." The transparent floating roof remained, but as a single rather than triple span. The columns that would have been necessary to hold up the ripples of the roof could be removed, giving the building more internal space.

The idea of a floating roof meant that it could be put up early. This was a trick Davies learned on another megaproject, the Millennium Dome

The three-wave roof concept was instead used at another RRP-designed project: Barajas airport in Madrid. Barajas offers an illuminating contrast to the problems faced by T5. Heathrow is cramped by lack of space; Madrid took out a hill to provide the necessary room. T5 has had to be vertically compressed – it fits into a 40 m high space and is 400 m long; Madrid is 1.2 km in length. As Davies puts it: "If you were building from scratch, you wouldn't have chosen the T5 location for all the tea in China."

The idea of a floating roof at both T5 and Madrid is significant. It meant that it could be put up early, allowing contractors to work on internal stages of the project in dry conditions. This was a trick Davies learned on another megaproject, the Millennium Dome: "We built the envelope of the building early, while they were still procrastinating about what would go below it."

RRP has taken this notion further, so that there are a number of "last responsible moments" at T5. For example, the envelope of the building has to be ready early in the design process; retail layout will not be fully detailed until later in construction; and signage placement will be decided close to the opening of the terminal. Davies says: "The dome taught us a way of moving off first on parts of the project to gain time. It was a good lesson for me."

Gaining time is important for a project as complicated as T5. Airports have so many variables that inertia can easily drag the process back. On T5, Davies has to get 43 stakeholders in the airport to approve each part of RRP's design. In a typical office building, Davies would require approval from about five.

Stakeholders at T5 range from BAA office staff, who have certain office requirements; BA, the occupier; Customs and Excise … even the trolley team have a say, which is just as well, as they must ensure the flow of 10,000 pieces of equipment to meet passengers needs. Inevitably, this is a bureaucratic, over-managed process, and not without its problems: "As a practice, we are used to leading a team, taking the initiative and providing vision. Sometimes this is difficult to do at T5. The system tends to knock off the sharp, brilliant corners. Sometimes the system tends to overpower the people," he says.

However, Davies is quick to point out that there is little other way the client could operate. "It is a technical and administrative machine. It is not a criticism, just a fact. Airports are immensely complex. Decisions take longer than with other clients, but then their projects are so much simpler. BAA has a project that is vital to Heathrow's standing as a world transport hub."

Davies appreciates that T5 is a project where teamwork is essential, not only with the client, but also other architects and different members of the supply chain. Within his own practice, the directors have a meeting every Monday morning, where they work through each other's projects. Most practices have a director in charge who effectively just looks after their own schemes; at RRP it is more of a group effort. Davies is in nominal charge.

We are used to providing vision. At T5, sometimes the system tends to overpower the people

But T5 is too big a scheme for one practice to take on. RRP has had to work with several other practices in different parts of the projects. For example, Chapman Taylor covers much of the project's retail; YRM is BA's architect-in-residence; HOK covered the rail side, which includes the extension of the Heathrow Express; Pascal + Watson worked with RRP on the buildings; and Din Associates worked on the interiors. For much of the project, these architects were co-located in the same room. "RRP is the lead architect, co-ordinating all of those other practices. It is important to work together – for example, I don't know as much about retail designs as some of the other guys," says Davies.

RRP has worked in tandem with contractors, too. Sensitive to BAA's £4bn budget, RRP wanted to keep the cost of the roof down and so made use of "concurrent engineering". This meant that the practice worked in collaboration with steel contractor Severfield-Rowen. "We sat down together in February and March of 2000 and explored ways of developing the design of the single span, best value roof. This way we knew what we were doing was achievable and could be costed by the contractor." As a result, it was easier to find methods of keeping costs down, such as preassembly and prefabrication.