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Here we have one of the key development challenges of the present decade. How do you provide affordable key worker housing in central London? Oh, and build it at high density on awkward brownfield sites at the same time? Friendship House hits all these buttons. The newly completed development provides relatively cheap accommodation for 179 people, most of them young, single newcomers to London, and 30% of its rooms are reserved for key workers. Its location is Borough, just south of Waterloo Station in central London. It fits into an awkward, landlocked site overshadowed along one side by a high, busy and noisy mainline railway viaduct leading into the vast station. And it has been built at the extraordinarily high density of 969 bedspaces per hectare.

The developer is London Hostels Association, which was set up in 1940 to rehouse Londoners made homeless by the Blitz. The architect is MacCormac Jamieson Prichard, a practice with a strong track record of designing low-cost public housing and making the best of difficult sites.

In basic concept, Friendship House is perfectly simple and straightforward. It consists of nearly identical bedsits, each with its own en-suite bathroom, lined up on either side of a central corridor – just like a university halls of residence.

But it is the way in which the 179 bedsits and supporting common areas have been arranged around the difficult site that is clever. Packed in at such a high density, they retain a palpable sense of comfort, daylight, space and amenity – with next to no noise or vibrations from passing trains. As senior partner Richard MacCormac puts it: "The principle here is that the building should respect people and be carried out with care. And we have put as many of the project's resources as possible into the common areas so as to make them really beautiful."

The basic site layout entailed extruding the rows of identical bedsits as one continuous strip and wrapping it round the perimeter of the site. The hollow core of the layout is a landscaped garden which half the bedsits and three of the common areas overlook. The awkwardly angled corners on each floor are taken up by shared kitchen–diners, which also look out over the central garden.

The wraparound strip of bedsits steps up from four storeys to six alongside the railway viaduct. As it does so, the strip switches in section from dual aspect, with bedsits either side of central corridors, to single-aspect along the railway flank, with just the corridors fronting the viaduct. In this way, the blank-sided tallest block acts as a buffer separating the living and amenity spaces from the noisy trains.

Looking more closely at the site layout, it turns out to be more of a spiral than a wraparound configuration. The four-storey block away from the viaduct stops short of the site entrance to provide a forecourt. It then drops down to two storeys and veers sharply sideways into the heart of the site. This wing contains a television room above a pair of common rooms, two of which overlook the central garden through a window wall. As it cuts E E across the south side of the site, this lower wing also admits sunlight into the inner courtyard.

Though it is a practical solution, the angular spiral configuration can be baffling to visitors. To give a clear sense of orientation, a straight walkway paved in dark-green slate cuts cleanly through the ground floor on a near-central axis. Starting at the main reception desk, it links all the common spaces and even includes a scenic interlude as it passes through the inner courtyard garden.

The detailed design continues to emphasise amenity and comfort. The secluded inner courtyard is attractively landscaped with a slightly oriental feel. Across it stretches a rippling pool with a small splashing fountain. The scene is so peaceful that it seems inconceivable that just a few yards away mainline railway trains are hurtling back and forth.

The external walls are all faced in white-painted render, which fairly bounces sunlight and bright daylight around the tightly enclosed courtyard. The downside is that they give the buildings a slightly stark, clinical appearance, though this is subtly improved by narrow horizontal bands of turquoise mosaic at each floor level. The external wall overlooking the railway viaduct is sheathed in zinc shingles with tiny windows to the corridors behind, and this presents a dramatically intriguing image to passing commuters.

As for the bedsits themselves, these measure a compact 12 m², including the en-suite shower room – that's about 1 m² smaller than a typical student residence. Here again, careful detailed design has come into play. The window has been placed to one side, where it washes one wall in daylight while giving the bed on the other side more seclusion. The desk and closet have been purpose-designed to fit snugly into the tight space. And the shower room has been prefabricated by RB Farquhar as a steel-framed pod on an L-shaped plan, with a fitted closet filling the corner gap.

Developing living accommodation at such high density so close to a mainline railway viaduct has put a premium on the suppression of noise and vibration. For a start, the entire building sits on top of anti-vibration bearings. And to provide sound insulation between bedsits, they have been separated by walls of dense concrete blocks 215 mm thick and finished in wet plaster. The cross walls between rooms are in fact loadbearing, as they support the precast concrete floors and are built up off strip foundations.

Since its opening to the first residents last September, Friendship House has proved remarkably popular, with London Hostels Association claiming that it receives some 400 applications a week. To Russell Handy, a Eurostar employee and one of the residents, it scores well on several counts. "It is close to where I work at Waterloo Station; the rooms are amazingly quiet, and there are people from all over the world, so it's like a micro-London," he says.

Not only is Friendship House regarded by London Hostels Association as its flagship development, but it sets a fine standard for single-person housing in Britain's city centres.