It wasn't quite a camel through the eye of a needle, but it was close enough. The challenge set for the construction team from prefabricated timber panel build specialist Framework CDM was to crane the component pods of a new penthouse flat in central London up six storeys. They then had to be ever so gently lowered into the roof space gap between the chimney stacks, with total clearance of 30cm – and the biggest of the units weighed 18 tonnes.

Asked whether this was just another day in the life of Framework, managing director Dave Madgwick was refreshingly frank. “Without any shadow of a doubt,” he said, “technically it was pushing the envelope of what we could do to the absolute max.”

It can't have helped stress levels that the project also took place in a pretty prominent spot. The penthouse sits atop one of the capital's prime addresses, Albert Court in Knightsbridge, giving a highly prized vista of the Albert Hall.

Having stretched the technical envelope to the max once, Framework is keen to do it again

Framework was selected to provide its prefabricated panellised solution by developer Fieldvalley Ltd. In fact, it was contracted to supply two penthouses for the building at a total project value of more than £1m. The first, which is the one now in place, is a 1,650ft2 two-bedroom flat, comprising five pods. The second, on which there has been a slight hiatus (of which, more below) is a three-bedroom 2,450ft2 apartment, made up from 11 separate structures.

“It's a prestigious and very busy part of town, so the developer wanted to minimise disruption,” said Madgwick. “Clearly there was some upheaval. The first apartment involved a week of lifting and we had to close off the street while this was going on. But taking the prefabricated approach caused far less disturbance than conventional building on site would have done.”

Framework also had to get to grips with “challenging design” issues. The penthouses were devised by architects Gerald Wolffe Associates to be in keeping with the Grade II-listed nineteenth century, Norman Shaw-style building below. The flats had to follow the curve of the frontage and include details (such as huge, arched timber-framed windows) to match the imposing structure, which includes corner turrets and four-storey colonnaded loggias.

The apartment involved a week of lifting but this created less disruption than a conventional building would have done

Each of the floor, ceiling and wall cassettes for the pods was designed and engineered using CAD and made up in Framework's Bedford plant. The company's wall panels comprise a framework of 140mm CLS timber, 15mm of Bitroc sheathing, 9mm of OSB3 for the internal lining, 140mm of mineral wool and vapour barriers. They also include cam locking fixing systems, lifting plates and first fix electrical containment. The roof cassettes have a core structure of press web joists and various types of laminated engineered timber, glulam, Parallam or Timberstrand, depending on the application. They also include 18mm and 9mm flexible plywood for the external and internal sheathing respectively, mineral wool insulation, vapour barriers and breather membranes.

These panels, says Framework, are strong and rigid in their own right and, in most of its buildings, form the core structure. Because of the unique demands of the construction process at Albert Court, however, the penthouse pods were a hybrid design, including a reinforcing steel element.

“The height of the lift did create special considerations, so we effectively used a steel cradle around each of the pods to provide additional stiffening,” said Madgwick.

External fittings,including roof tiles as well as the windows, were also pre-fitted, as were some second fix internal items.

The flats had to follow the gentle curve of the existing frontage

Once completed, the pods were transported to site on extended flatbed trucks – a testing logistical exercise in itself.

“Clearly it wasn't a job to do during Friday night rush hour!” said Madgwick. “To minimise congestion and the amount of escorting, it needed to be planned in detail.”

Because the pods were fully sealed and weathertight, the risk of water ingress into the existing building often associated with roof extensions was eliminated. As they were being fixed to a concrete roof, there was also no need for any additional fire prevention systems on the underside.

Despite the logistical complexities and slim margin for error on the project, the first smaller penthouse was completed on time and with no major hiccups. The hiatus with the second was outside Framework's control. A serious fire broke out in the apartment below and the penthouse pods had to be put into storage. It is hoped, though, that they too will soon be swinging into position high above the streets of Kensington.

And having stressed the technical envelope to the max once, Framework is keen to do it again.

“It's been a very valuable technical exercise for us and we're keen to do a lot more of this kind of roof extension work,” said Madgwick. “In fact, as a result of this project, we're already getting approaches to do others.”