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The result of Powell's big experiment is the blueprint for a new medium-sized hospital that will do away with the traditional outpatient, accident and emergency and other specialist departments. Instead, its 214 beds will be spread through six agglomerated departments with strange names such as "major assessment centre" and "expert consulting centre". Its architectural design by Avanti Architects and HLM Design is no less distinguished: it will feature an airy mall linking a series of courtyards and an oval civic plaza at the front leading visitors up to the main entrance.

The revolutionary new hospital is being planned by Powell's project team at the Central Middlesex Hospital NHS Trust to replace its obsolete building complex in north-west London. Known as BECaD, which stands for Brent Emergency Care And Diagnostic Centre, it is being procured by means of a PFI, with the French contractor Bouygues selected as preferred partner last December.

BECaD has two significant things going for it that could establish it as a model for a district general hospital. The first is that the scheme comes with a prize pedigree. An earlier project, also by the Central Middlesex Hospital NHS Trust, was the ACAD Centre, (Ambulatory Care And Diagnostic Centre), completed in 1999. ACAD established a new healthcare building type, the diagnosis and treatment centre, of which another 40 are now being developed around the country. Exclusively dedicated to elective or routine operations, such as hip replacement or eye cataract removals, it increases throughput by segregating these cases from severe and emergency cases, which take priority but are less predictable and hence more disruptive to workflow.

As designed by Avanti, ACAD is acclaimed as a shining but all-too-rare example of a healthcare building that genuinely raises the spirits. A photograph of its central mall – clean-cut, suffused in daylight and flanked by luxuriantly landscaped courtyards – has graced the cover of Tony Blair's architectural manifesto Better Public ºÃÉ«ÏÈÉúTVs.

The second strength of BECaD is that its medical concept, architectural design and role as an agent of urban regeneration are all products of an extended period of concept development and consultation. In contrast with other PFI hospital projects, the process began with a six-month exercise of "clinical redesign" by Powell's 13-strong project team. This entailed three "table-top simulation exercises", in which a typical day's workload for the hospital was tested out using the proposed new system, with up to 30 doctors, nurses and administrators playing out redefined roles.

After that, and before PFI bids were invited, Avanti was given 12 months to draw up a public sector comparator and produce a 3D architectural design. During that period and the following six-month PFI bidding stage, the project was treated as a pilot project for better healthcare design, involving the Prince's Foundation and CABE in some 20 consultation exercises.

But why turn the traditional hospital concept upside-down in the first place? Powell paints a disturbing picture of hospitals growing in size and complexity and becoming more dysfunctional as they do so.

"There are several issues involved," he explains. "Medical teams are becoming ever more specialist, so you need to gather larger collections of them together in fewer, more centralised locations to give a comprehensive service. Specialisation also brings problems of co-ordinating how the various systems and services fit together. At the same time, the drive to centralisation makes hospitals more remote from support services in local communities. Then there is pressure from the European Working Time Directive, which drives down the workload of junior doctors from 80 to 48 hours a week, so we need to make more efficient use of their limited time."

As in the earlier ACAD project, the key objective behind the clinical redesign by Powell's team was to streamline the patient pathway through the hospital. Medical teams and their departments were then reorganised around these workflows.

"The idea was to rearrange teams into super-teams that merge together hospital services and primary care in the community," says Powell. "And we have graded services according to severity, not according to medical specialism." The aim was not just to improve co-ordination within the hospital, but also with aftercare services in the community after the patient has been discharged home.

This means the traditional accident and emergency department has been split into an urgent treatment centre for minor accidents and a major assessment centre for severe cases, which has been merged with critical care to form a large 100-bed acute unit. The hospital's seven specialist departments were consolidated into an expert consulting centre serving both inpatients and outpatients. The other main departments are a 24-bed major elective surgery centre, for non-emergency operations requiring longer stay than catered for at ACAD, and a 90-bed step-down service, where patients can recuperate for a few days before being discharged.

As part of the PFI bidding process, the building went through a second phase of architectural design, in which Bouygues' design team of HLM Design and Avanti refined Avanti's original public sector comparator scheme commissioned by the NHS trust. "We took the bold move of shifting the operating theatres upstairs, along with support services, staff and education, and this helped free up the ground floor for public areas," says Avanti director Claudia Bloom. And Bouygues proposed putting a 400-capacity car park beneath the building, which would provide secure parking for staff and an expansion zone on part of the site that would otherwise have been used up by surface car parking.

"We also put a major public zone in the heart of the building and a public route that links directly into ACAD at the back of the building," continues Bloom. A cross axis route on two floors leads through a series of linked blocks wrapped around courtyards, which – as in the ACAD building – brings daylight, sunshine, landscape and a sense of the outside world into the heart of the complex. At the intersection of the two main circulation axes lies a top-lit atrium, which forms the main public zone of the hospital and is served by a reception, waiting area, cafe, restaurant, shops and lifts and stairs to upper floors.

Beyond the hospital building itself, there was an agenda of urban regeneration and design, which was the particular concern of the Prince's Trust and grew in importance as the project progressed. "The traditional hospital turns its back on the community," explains Bloom. "Yet a large hospital ought to be an agent of regeneration in economic, urban and design terms. There was a desire, first laid down in a masterplan by the Park Royal Partnership, to create a new urban heart for the area. This was sorely needed, as Park Royal had become a distribution centre of large sheds. So we set out to create a urban space in front of our main entrance. Our building curves out and grabs people and pulls them into the site."

The civic space in front of the hospital takes the form of a wide oval piazza with a bus drop-off and pick-up point. A three-storey crescent of hospital buildings fronts one side of the piazza, with a long curving canopy over the pavement in front to guide visitors to the main entrance. The other side of the piazza is earmarked for related developments that could be built later, such as key-worker housing and a medical technology centre.

Like the ACAD Centre before it, BECaD is on target to revolutionise medical care and raise the standard of medical architecture at the same time. When completed in December 2005, the chances are we can expect to see it on the cover of the next edition of Tony Blair's Better Public ºÃÉ«ÏÈÉúTVs manifesto.