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Fifteen years after Richard Rogers Partnership submitted designs for Heathrow's Terminal 5, construction is finally under way. The £3.7bn terminal – one of the most expensive projects in Britain – is a pragmatic response to rising passenger numbers, and it is also intended to maintain Heathrow's claim to be Europe's premier airport in the face of competition from recent developments at Madrid and Amsterdam. Right now, however, T5 is functioning best as an example of just how difficult it is to get a project of its size through the UK's planning system. And, more ominously, it may demonstrate just how difficult that system makes it to come up with the right design for such a project.

The sequence of events between the first sketches for T5 and work beginning on site is well known – they are arranged on a convenient timeline over the page if you need to refresh your memory – but the central event was a public inquiry at which, for three years and 10 months, local residents fought a vehement, entrenched delaying action.

Critically, that inquiry locked BAA into the scheme it submitted; on approval, it could not alter its size or appearance without risking another inquiry – not an appetising prospect. This was a problem because the aviation industry underwent a period of rapid change in those 15 years: this affected everything from baggage-handling technology to security requirements to the economic well-being of the carriers. The result was that BAA found itself between a rock and a hard place. It had to make the terminal fit the industry's new requirements, it had to make it adaptable enough to accommodate future changes, and it had to do this within the parameters set by the inquiry and within the budget. BAA passed the problem to its design team, no doubt with a prayer in its heart. If they could not find a solution to the simultaneous equation, a new design would have to be resubmitted for planning.

The original scheme envisaged a colossal rectangular terminal building topped by a roof that surged over it in giant waves. The effect was to shield the building's towering glazed facades on three sides, and on the front elevation, to oversail the approach road and shelter car-borne arrivals. This canopy was held aloft on four rows of tree-like steel columns that rose up out of "canyons" – two lightwells that ran the length of the terminal, effectively dividing it into three long, thin buildings. "The building becomes the backdrop for the airport drama because of its immense scale," says Mike Davies, the partner at Richard Rogers Partnership responsible for the scheme.

After the delay caused by the public inquiry, BAA had to satisfy itself that the design was still appropriate. "We had to go back over the design to be comfortable that we were trying to solve the same problems and to be sure that this design was the best solution," says Mike Forster, T5's design and development director. "We are in incredibly uncertain times. We expect BA to be the terminal's first occupier, but if it goes bust the design has to be robust enough for another carrier." This is a sentiment echoed by Dennis Austin, head of the roof design team for Richard Rogers Partnership. "BAA were always hammering on about futureproofing," he says.

Rogers' initial design, with its 384 × 165 m roof supported on serried rows of structural columns and floorplates separated by canyons, was far from flexible. "The design submitted to the public inquiry was based on a multiple-span roof with a zoned building below. Sitting at the public inquiry we realised that it wasn't as flexible as it could have been," Davies says. "It became an issue of where to site the roof-support columns and the impact of the structure on the building's operation."

Austin adds: "The public inquiry locked us into the colour, the cladding materials, a maximum height, the facades' transparency, a 10% rooflight area and the dimensions. It was about the building's shell, its overall volume and its visual impact."

The task was further complicated by the difficulty in defining the form of the interior, a process Davies describes as "wrestling with serpents". He explains: "There were 43 different stakeholders, from immigration and customs to baggage, retail and trolleys, all independent – it was like trying to weave a Turkish carpet."

In the interest of flexibility, the concept of canyons breaking up the terminal's floorplates was gradually abandoned. "The problem was, we ended up with three buildings. The canyons brought light and air into the terminal but they conflicted with virtually everything else," Davies says. To improve flexibility, the canyons were shifted to the perimeter. "We moved from the concept of canyons to a single floorplate – it was very difficult to do, we had to let go of things we were passionate about. There was a moment when we put in the bin a lot of the things we had been working on."

To increase the building's flexibility still further, and to allow the terminal's design to progress independently of the interior, the roof structure was isolated from the layout of the interior; instead, it simply spanned the entire 165 m. "The roof is effectively a bridge; its span gives a loose-fit, flexible floorplate offering enormous flexibility in how the building can evolve," says Davies.

Throughout the evolution of the design, BAA kept a beady eye on cost. "We were looking for the highest value solution, and value-engineering is the way to design value in," says BAA's Forster, returning to his favourite theme. Davies is less tactful: "We had the hounds of hell monitoring cost – the costing process ran alongside the design development."

There final hurdle the team had to clear was obtaining detailed planning approval from Hillingdon council. The designs were submitted in the summer of 2002. Everyone held their breath. There was an issue with the roof: the public inquiry had allowed the scheme to go ahead on the basis of a waveform roof. But would the team's new value-engineered single-span, single-wave roof fit this definition? And would the new transport interchange, with its innovative rooftop forecourt prove acceptable (see the caption on page 36)? To the relief of all, the planner's response in January 2003 was positive. At long last construction could begin.

The first part of the terminal building to be constructed will be the roof. Erection is programmed to begin on 27 October this year. But the design team's job is far from over: "We've detailed the roof out completely, but the internal layout is still evolving," says BAA's Forster. For Davies, the finishing line is in sight. with work now progressing on finalising the interior layout: "The design is being rinsed out with the client with input from [future tenant] British Airways," he says.