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Even in a city famous for its concentration of huge commercial buildings, Birmingham's Royal Mail sorting office breaks several size records. For a start, it is the largest building in the city. With a footprint that covers two entire blocks, or 1.6 ha, it rises to five storeys and packs in 65 000 m² of floor area.

That's not all. Beginning next month, the building will be converted and extended, increasing the floor area to 100 000 m² and making it the largest converted building in the UK – the same size as the entire Brindleyplace development.

On top of that, its developers claim it will be the largest mixed-use building in the country, bringing together double-height office floors large enough to house call centres, a huge shopping arcade, cafés and restaurants, a 200-bedroom hotel and 140 dwellings, all under one roof.

The £50m conversion project, christened the "Mailbox", is the brainchild of Alan Chatham and Mark Billingham, formerly development director and property consultant respectively for Brindleyplace. Their development company, Birmingham Mailbox, acquired the sorting office last April with funding from the Royal Bank of Scotland. Planning permission followed in October.

The architectural design, by Associated Architects of Birmingham, is straightforward and modern, and complements the utilitarian 1970s building. It revolves around a 200 m long street or arcade carved through the whole length of the building. The arcade will be lined by shops and a health club, with offices at the higher level. The 140 dwellings, built for Crosby Homes, will be at roof level, some converted from the existing top floor, but most erected as two-storey maisonettes on top of the existing roofslab.

The entire construction project has been let as a design-and-build contract to Tarmac. Demolition will start next month, with shell-and-core standards being completed in May 2000.

Chatham comes across as remarkably unfazed by the scale of the planned alterations. "There isn't a huge amount of work needed to services and structure," he says. "Most of the works are to the external shell. And it's a massive building with big elements that can be worked on simultaneously. Even cutting a street through the interior is a non-issue, as the existing structure is just a big Meccano set with a steel frame on a 12 x 12 m grid. Lateral restraint is supplied by existing lift and stair concrete towers." At either end of the scheme, Mailbox will give two open spaces back to the cityscape. A canalside piazza will be created, where cafés and restaurants can spill over up to the water's edge, and a pedestrian suspension bridge will span the canal.

"This is the only piece of Birmingham's canal network that is orientated to the South-west," enthuses Chatham. "It's a very special place where there is a combination of water and sun, and the sun shines on you all afternoon." In Birmingham, even huge, deep-plan developments must now have their attractive outdoor spaces. How times have changed.