Making history

Gillian and Tom are among the 24 families that have been allocated a flat at Poplar Tree Gardens, an apartment block notable not only for the generous size of its flats but also because it is only the second in the country to be built entirely of factory-made units. "I wouldn't have known if they hadn't told me," Gillian says. She appreciates the advertised advantages of prefab housing, such as the absence of teething troubles. More surprisingly, she likes its individuality. "It's modern and different," she says. "Too many of these new housing estates look the same."

The £2m development does indeed look sensational. Clad in cedar and terracotta tiles, and articulated with balconies and walkways, only a trained eye could spot the significance of the horizontal and vertical banding on the elevations of the L-shaped building. They are the clues to the block's origins as 48 identical, prefabricated boxes stacked on top of one another.

Designed by architect Cartwright Pickard and manufactured by Portakabin stablemate Yorkon, Poplar Tree Gardens is a follow-up to the team's highly acclaimed Murray Grove development in east London, a pioneering 1999 project for the Peabody Trust housing association. The 30-apartment scheme showed how "volumetric" techniques widely used to build hotels and fast-food outlets could be adapted to provide high-quality homes.

Back in York, the technique's latest client, Yorkshire Housing, is as delighted as Ms Sanderson. The project has delivered apartments that are 10% more spacious than social housing norms, in about half the time it takes to build a traditional development, and still meet the tough cost guidelines set out by the Housing Corporation.

There have been other benefits as well. "We haven't had a single complaint from the neighbours," says Peter Atkinson, Yorkshire Housing's group property director. "You'd expect a few, especially in a residential area with a school across the road." With most of the noisy, messy work done in the factory, almost the only thing the site's neighbours had to put up with was a stream of lorries ferrying modules to the site.

The project began when a group of York councillors visited Murray Grove and decided their own city should have a similar development. For the client and the council, which co-funded the scheme, it was a chance to display their forward-thinking credentials and express regional pride by employing the services of local firm Yorkon, whose factories are less than a mile from the site. For the architect and Yorkon, which acted as manufacturer and design-and-build contractor, it was a chance to refine techniques developed at Murray Grove.

"The level of sophistication has increased," says James Pickard, director of Cartwright Pickard. "On Murray Grove, we were almost learning as we went. This was a slicker, faster process."

The programme, from contract to handover, took 14 months, which the client estimates to be a full year less than a traditionally built scheme.

Pickard admits that the experimental nature of Murray Grove meant that none of the firms involved made any money on the job, and that his practice produced far more drawings than it otherwise would. Things have definitely improved second time around, though. "We got the drawings on time this time," laughs Keith Blanshard, Yorkon's director and managing director.

As with the London development, the steel-frame modules were fully fitted out in Yorkon's factory, right down to the smoke alarms and carpets. And although budgetary constraints mean the fit-out is more utilitarian than at Murray Grove, the York scheme contains a number of improvements. The most significant is the use of fewer, larger modules: two for each apartment, compared with three at Murray Grove.

The use of larger modules allows more electrical and mechanical work to be done in the factory, and cuts down on the number of interfaces between modules, which have to be sealed on site. "Every time you've got an interface, you've got a risk area," says Blanshard. "If you've got a light switch in one module and the light is in another, you've got to have an electrical joint. If they're both in the same module, it's a whole lot easier."

The York modules also have built-in conduits hidden in the floor, allowing residents to install additional wiring at a later date – something that could not easily be done at Murray Grove.

Each module is a structural unit consisting of wall, ceiling and floor panels. It is not exactly rocket science: each panel is a straightforward sandwich with a top layer of steel, a bottom layer of plasterboard and a filling of polymer foam.

Load-transferring columns concealed within the wall panels brace the units when they are stacked together, meaning that no additional external frame is needed. The modules are craned into E E position and aligned with conical locating pins on the units below, before being bolted together by operatives in cherry-pickers. Following consultation with workers, the assemblies have been redesigned and repositioned. "We've listened to the guys on site," says Blanshard. "We've moved the bolts to make them more accessible." As a result, the number of modules assembled in a day increased from eight at Murray Grove to 10 in York.

Not every element of the building was executed in the factory. Tight production-line scheduling at Yorkon's plant – which also turns out mini-supermarkets for Tesco and drive-in restaurants for McDonald's – meant there was no time to wait around while cladding details were finalised. Instead, scaffolding had to be erected on site and the job done at the mercy of the weather. "The cladding not going on in the factory is the biggest single challenge that was not met," admits Pickard. "We have to hold our hands up and say we didn't get there quickly enough."

The dramatic, perforated steel-clad stair- and lift-tower was built from lightweight blocks and insitu cast concrete stairs, since one-off elements such as this do not warrant the expense of setting up production lines. With the unrendered blocks simply painted over, this is one of the few parts of the building that lacks finesse. Another is the wonky saw-cuts and drips in the cedar cladding, which reveal the drawback of reliance on human craftsmanship. A series of ugly craters (due to be filled after Building's visit) in the underside of the access decks show where the precast concrete decks were core-drilled on site to allow rainwater pipes to pass through – a task too difficult to perform accurately at the factory.

Elsewhere, though, the machine aesthetic has been seen through with great success. The ground-floor modules, clad in clip-on terracotta tiles rather than cedar, serve as a useful show-and-tell of how, in volumetric construction, the architect is subservient to the components. The entire elevation is laid out around the dimensions of the tiles which, being a standard commercial product, could not be altered. Hence the size and positions of windows, doors and fan grilles – even the modules – are all dictated by the tiles.

"We went through a very laborious positioning and setting out to ensure that the tiles didn't have to be cut," says Guy Gregory, project manager at Yorkon. "You wouldn't believe the turmoil we went through to agree the type of fan grilles and their position."

Gillian Sanderson may not be particularly bothered by the position of her fan grilles, but she's delighted with everything else. "I'm really pleased with it," she says. "I didn't have to do anything when I moved in and everything works properly. All the little things impressed me – the smoke alarms, the locks on the kitchen cupboards so that children can't get in them …" When pressed, she can think of just one niggle: "I've got a bit of a problem with my TV reception." Oh well, don't we all?