As city-centre sites get scarcer, developers are getting ideas above their stations, putting offices on the market – literally – and giving a whole new meaning to living on the river. Victoria Madine looks at the rise of the stack development
Imagine cities where a home over a petrol station, fire-station or school is as common as a flat above your local grocer or curry house. While you are at it, imagine shopping in supermarkets above railway bridges and visiting friends in flats built on stilts over a health club. If you can't, don't worry – it could soon become reality.

As land values in the UK's towns and cities escalate and planning policy guidance pushes local authorities to increase brownfield and mixed-use development, developers are increasingly coming up with novel ways of exploiting air rights over existing structures. Petrol company Texaco has revealed plans to develop retail, housing and office hubs above its service stations. And housing association the Peabody Trust is planning to build an inhabited bridge to span the mouth of the River Lea in east London. These are just a handful of examples of developers' recent innovative approach to the shortage of city-centre sites.

The concept of piggyback structures or overlapping developments is far from new. London Bridge supported a row of shops and flats until its destruction in the Great Fire of London in 1666. Flats above shops have long been the norm in city centres. In the late 1980s property boom, developers extended the concept by creating large-scale developments over existing structures, such as the Hatfield Galleria shopping mall over the A1(M) in Hertfordshire. Many major rail terminals in London were capped by offices or shopping centres and the ceiling of Manchester's Victoria station became the floor of the Manchester Evening News Arena.

But these developments were one-offs and focused on exploiting large spaces in dense metropolitan city centres. Now developers predict that stacker-style, multi-purpose buildings will become commonplace in smaller towns as well as cities. For example, now that the days of building low-rise sheds on greenbelt land are long gone, retailer Tesco is looking to build a supermarket over a railway bridge in Gerrards Cross, Buckinghamshire. Tesco and other retailers are grappling with the combined effects of recent planning guidance papers PPG3 and PPG6. Introduced last year, PPG3 sets targets for brownfield site developments, while under the sequential test introduced in the 1996 revision of PPG6, retailers have to first consider town-centre sites.

Gerrards Cross, a commuter town in the greenbelt, lacked suitable sites for a supermarket, so Tesco proposed building on top of the railway that bisects the town. Tony Franklin, a planner with South Buckinghamshire District Council and team leader for Tesco's application, says that locals had mixed feelings about the idea, but adds: "One positive benefit is that we would be filling the gap created by the railway, which splits the town into two. The shop would provide continuous retail frontage."

Now developers are rushing to capitalise on other unlikely sites. "Landowners are beginning to realise what they are sitting on," says Russell Pedley, architect at Assael Architecture. "They may already have a building on their plot but they have rights to develop the space above it and they are going for it. And shifts in planning guidance are encouraging this mixed use." This new thinking is creating opportunities where flats would previously have been unthinkable. Assael is working with Texaco to design the biggest of its pilot schemes – a £35m riverside tower above a petrol station on London's Albert Embankment (see "Getting on top of things", above). Pedley says Assael has also received enquiries from developers about similar schemes in Manchester and Leicester.

Another previously unthought-of airspace with potential is above schools. Architect Pollard Thomas & Edwards is pioneering this idea with its proposal to build flats on top of a new primary school in Islington. Project architect Andrew Beharrell explains: "Schools are not occupied for many hours in the day. Many people live near a school – our development would be no worse than that. Schools are an expensive use of the land as they are all on one level. Doubling housing with a school is a solution for making better use of the land. It also means the school gets land at a reduced rate."

A report by the DTLR last February entitled Developing Additional Housing Above and On Non-Residential Sites concluded that non-residential land "has the potential to be a very significant source of new housing". The report says the process could deliver up to 2000 units by 2004 – as long as all the stakeholder groups such as RSLs, local authorities and developers are committed.

The very fact this report exists suggests the government is hoping that these processes will become a standard response to the country's growing housing crisis. Steve Thompson, director of developer Ridgewood Investments and head of the Texaco development project, believes this faith is matched by commercial realities: "With a low-rise development you cannot always warrant the cost of the land. It would be nonsense not to utilise the masses of space available over existing structures."

Improvements in building technology are creating more opportunities for the use of stacker-style developments. Transfer structures made of prefabricated units are lighter than traditionally built structures and can be craned into place, thus reducing the need for heavy bridging or transfer structures and leaving the host building's occupants undisturbed. "If you can reduce the need for the bridging structures by using modular technology, you can halve the building costs – the system is particularly appropriate to the issue of air rights development," says James Pickard, director at architect Cartwright Pickard. Lightweight modular technology also reduces the need for supportive columns, making it particularly suitable for building over shopping centres, where floor space needs to be preserved. Cartwright Pickard is working on modular technology for the River Lea inhabited bridge (previous pages).

Pickard says more and more clients are asking about these types of structures. "We've been approached by hotel companies that are finding it difficult to find low-cost accommodation for their workers – one company is even interested in creating a structure supported by stilts that will sit over an existing structure," he says. The government is also keen to explore the potential of this technology as a solution for housing needs for the police, key workers and even asylum seekers. As a result, the Home Office has seconded a senior manager to work with Cartwright Pickard to find out more about the potential afforded by modular technology.

Developers agree there is a definite and growing role for stacker structures in the future, but even its most proactive supporters warn it is not a simple solution. Dickon Robinson, director of development and technical services at the Peabody Trust, says more research into the method is needed. "I can see the market for this type of development building up steadily. But we need more pilot projects to make sure we understand the challenges." Robinson believes a very long-term view needs to be taken of these developments, especially those with a retail element. "You've got to consider the future: will there still be demand for a shop in 20 years time? Can the site be redeveloped? A lot of warehouse-style shops have been built on the assumption they will be dismantled at some point in the future. It could be a problem to recycle these buildings while retaining the housing or whatever else may be above it," he says.

Richard Coleman, a planning consultant, points to the high potential cost of developments over infrastructure. "Station developments have to be enormous to pay for the extra costs of the transfer structures. But it is not always appropriate to have a large development, and a structure like Clapham Junction would present a problem – trains move through at 30 to 40 mph. A transfer structure that can work around these challenges would be enormously complicated and costly. You have a limited time to work and you can't close the station."

However, Coleman believes there is scope for more development over railway lines: "Railtrack is an ambitious developer. There is potential to build over Victoria station, and Waterloo has similar possibilities." Sure enough, Railtrack has also just announced a £600m redevelopment of Euston station, which will involve building over the railway tracks.

For Ridgewood Investments' Thompson, recycling developments is less a problem than the attitude of some local authorities towards stack developments. "Unfortunately, local authorities don't always realise how high the density of affordable housing needs to be to make economic sense to developers." Thompson points to a conflict between central government's enthusiasm, and local authorities' reluctance to support multipurpose "add-on" structures. "Developers are piggy in the middle. We need a general policy to support these types of developments. It's a question of breaking the mould and changing the way local authorities think."

Most agree that the rationale behind stacker developments is so clear that the difficulties will be overcome. As Coleman says: "Developments like this are essential as part of bringing life back into the cities. We will see a lot of developments of this type."

Getting on top of things: proposed projects

  • Architect Will Alsop of Alsop Architects has proposed an outlandish but simple idea to encourage developer Hammerson, the owner of historic Spitalfields market near the City of London, not to destroy the rear half of the market hall (pictured). Commissioned by the Spitalfields Market Under Threat campaign, Alsop’s speculative idea suggests there is no need to slice into the market hall or bury the archaeological excavations in the open space just behind it. Commercial space can be created by jacking office blocks high up into the air on stilts, leaving clear open space beneath. That way, the developers get their floorspace, and the residents and visitors get to keep all of their market hall, amd a sheltered archaeological lower level could lead into the building’s unexploited basement.
  • Russell Pedley, architect at Assael Architecture, is designing petrol company Texaco’s largest pilot project to develop office and residential space over its stations. The architect is working on a £35m riverside tower on London’s Albert Embankment. If this and Texaco’s other nine pilot projects prove successful, the idea will be copied at another 150 sites in UK cities. The riverside project would consist of two towers: one of 27 storeys, the other of 12. Achieving a balance of functions within the structures is essential, and the idea is to create a 24-hour building. The tower will have a shop on ground floor, then 3 or 4 floors of offices, then residential layers, with the offices acting as a kind of buffer zone between the flats and the petrol stations. Private space will be created by adding a glazed balcony area on each flat. The ground level service stations are all self-contained, fully-ventilated units in enclosed spaces, and a vapour recovery system will extract any noxious fumes that escape from the petrol pumps. For developments of this type, the design must address the potential conflict between vehicle movements and the need to access flats.