Haworth Tompkins' design for Coin Street on London's South Bank has been hailed as a new model for high-density inner-city housing. In this project review Coin Street Community Builders, Haworth Tompkins Architects and Davis Langdon & Everest look at the project's design costs

<B><font size="+1">Coin Street housing site B</font></b><B>Project</b>
High-density inner-city social housing scheme of 59 new homes

<B>Location</b>
South Bank, London SE1

<b>Architectural features:</b>

  • High density housing layout at 334 habitable rooms a hectare, conforming with revised PPG3 guidelines
  • Mix of large family and small household homes plus two corner shops
  • All homes have gardens, terraces and/or balconies overlooking a communal courtyard garden
  • Commercial car parking retained in basement to cross-subsidise housing
  • Architectural treatment responds to urban setting – hard brick screen on the outside, soft timber and landscaping on the inside
  • Low energy and sustainable design principles

<B>Cost and procurement features:</b>

  • Unit construction cost of housing at £1018/m2 is 36–104% higher than average social housing schemes, as a result of high density and quality specification
  • Lump-sum contract GC/Works/1 (1998)

<B>Construction period</b>
18 months to August 2001

<B><font size="+1">Client's brief</font></b>In 1997, Coin Street Community Builders, the not-for-profit community housing developer of London's South Bank, held a competition jointly with its housing co-operative arm to select the architects for its fourth housing development. The 0.87 ha brownfield site was an entire urban block behind the London Weekend Television Centre that had formerly been occupied by warehouses and was being temporarily used as a car park.
The brief for the housing was to fully exploit the site's potential for large family homes with individual gardens, while also providing for smaller households to create a healthy mix. All dwellings were to have private open space – gardens, terraces or generous balconies – but a shared garden was also seen as essential. Competitors were also asked to retain the lucrative commercial car parking within the basement, and plan for a neighbourhood centre for the southern third of the site.
From an initial list of nine young practices, six were invited for interview and four were selected to prepare outline designs. Each practice was assigned its own quantity surveyor from Davis Langdon & Everest but was allowed to select other team members. A communal briefing session was convened at the outset of the competition, and feedback to the competitors' outline proposals was given at halfway point.
The client considers that its decision to hold an architectural competition was fully justified by the range and quality of the ideas put forward. Haworth Tompkins Architects, a practice with no previous social housing experience, was unanimously chosen as winner, as its central courtyard scheme best addressed the challenges and opportunities of the site.

<B><font size="+1">Architectural design</font></b>The competition-winning design by Haworth Tompkins Architects is based on a quadrangle format, in which the site perimeter is occupied by a continuous strip of building leaving the centre free as open space. This configuration provides a suitably urban enclosure on the outside and a large open space measuring 57 × 60 m in the inside as a communal amenity, including children's play area.
Facing the two side streets are predominantly four-storey terrace houses, including an attic room set back from the street elevation and opening on to a generous roof terrace overlooking the courtyard garden. On the main frontage, facing the busy traffic on Upper Ground and the bulky tower of the London Weekend Television Centre, the building steps up to five storeys. Here, three-storey maisonettes are surmounted by two-storey maisonettes that are reached by a wide open deck overlooking the communal garden. Shops are placed at both corners of the block facing Upper Ground.
All the dwellings have ample access to external spaces. As well as entrances off the street, all the houses have private gardens opening into the communal courtyard. All the flats and maisonettes have large balconies, and every bedroom overlooking the courtyard has a balcony.
The elevational treatment acknowledges the dual aspect of the scheme, addressing the dense urban environment on the outside and the private landscaped green space on the inside. The street elevations are crisp brick screens with deep window reveals. Where the elevation steps back in the attic, the treatment is lighter with vertical pre-weathered zinc cladding.
The boundaries between the dwellings are emphasised on the front and rear elevations. The facades are designed as strong ordered backdrops against which occupants can personalise with net curtains and pot plants without undermining the whole composition.
The houses have a private buffer zone in the form of a raised pavement area separated from the public pavement by timber and stainless steel bin enclosures and railings. Each house also has a gated porch off the street with a large cupboard to store items such as buggies and online shopping deliveries.
The materials on the courtyard side have been selected to weather and mature with the landscaping. The infill cladding is a naturally durable hardwood from managed sustainable sources, Vitex Cofassus, which requires no preservative treatment or applied finishes. The same material is used for horizontal timber sunshades and balcony decking.

<B><font size="+1">Density</font></b>Conscious that high densities of children in social housing estates can cause social problems, the client emphasised the need for a wide range of dwelling sizes and quality of accommodation and environment rather than maximum density at all costs. Accordingly, the density was not prescribed in the brief but emerged out of the design. Even so, the resulting density of 334 habitable rooms per hectare is 59% higher than Lambeth's planning guideline at the time of 210 habitable rooms a hectare, and conforms with new government guidelines set out in revised PPG3, which proposes higher housing densities on urban brownfield sites.

<B><font size="+1">Low energy services</font></b>The buildings have been designed on low-energy and sustainable design principles that are easy for the residents to understand and use. Passive solar panels on the roof provide free hot water to residents for most of the summer and reduce demand on the heating system for the rest of the year.
Gas-fired condensing boilers with a heat-efficiency of up to 90%, heat-recovery and ventilation systems, low-emissivity double-glazing and a high degree of airtightness reduce energy consumption and bills approximately 30% while providing better internal air quality.
The basement car park is mechanically ventilated, supplemented by natural ventilation through the entrance ramp. The car park has not been allowed to disrupt the street elevations and house entrances, as they are ventilated by louvres discreetly located adjacent to the pedestrian entrances to the car park. Exhaust air is discharged at roof level five storeys above.

<B><font size="+1">Structural design</font></b>Many of the existing concrete pad foundations of the former warehouses have been retained and a new structural basement slab has been suspended from a combination of new and existing bases.
Four-hour fire separation between car park and housing was achieved with lightweight aggregate concrete and by widening the supporting columns. The structural concrete slab with a 125 mm floating screed insulation provides acoustic separation between car park and dwellings. In the central courtyard, the ground floor slab has been designed to support 650 mm deep planters. Everdure Caltite, a waterproof additive, was included in the concrete mix, making waterproof membranes unnecessary. The frame of the housing comprises reinforced concrete supported on columns in the party walls with dense concrete blockwork between.

<B><font size="+1">Cost and procurement</font></b>At £1018/m2 gross internal floor area, the costs of the residential element of the development are higher than would be expected for most social housing schemes (£500-750/m2). The main causes of this are additional costs associated with a high-density scheme (typically adding 10-20% to the cost of a project) and the level of specification required by the client. High-quality design features such as low-maintenance zinc roofing, extensive roof terraces and balconies and the use of low emissivity coated glazing together with energy saving components such as solar panels, and heat recovery add £200/m2 to the cost of the project. The costs of external works, at £120/m2 including preliminaries are also high as a result of the investment in communal facilities.
By contrast, the costs of the underground car park, at £450/m2 gifa, equating to £12,200 a space, are at the lower end of the cost scale (typically £370-570/m2). This results from the site already being part excavated, and from the limited requirement for retaining walls to one side of the site only.
Procurement was by competition for a lump-sum contract, based on measured bills of quantity. The contract was awarded to the Mansell Group in February 1999 for an 18-month programme. The GC/Works/1 (1998) form of contract with full Latham compliance was adopted by the client, as it offered an appropriate balance of risk between employer and contractor. The contract was later amended to incorporate many of the provisions of the PC/Works variant.

<B><font size="+1">A tenant's perspective</font></b><B>David Tootill, tenant member of Iroko Housing Co-operative:</b>
"The homes are very solidly built and the noise and heat insulation is exceptional. We are in the centre of London yet inside there is no traffic noise and we don't even hear our neighbours. Even on the coldest days we only need to have the heating on for an hour in the morning. In the summer the solar panels provide all our hot water – and there are seven of us living here. The bills are really low for a house this size.
"The use of space has been well thought out both inside and out. It's phenomenal to think how many people live here yet there is no feeling of over-crowding. The interior finish is excellent. The wood used on the stairs and in the other rooms can be varnished easily and cheaply, making buying floor covering as soon as you move in unnecessary.
"In the evening, when the lights are on in the central communal garden and the children are playing, it's like being on holiday – it has a Mediterranean feel. But I don't know how hardy the planting will be with all the children and footballs. The central garden has become so attractive to other neighbourhood children that we have had problems. Since then, we have introduced a kids' curfew and installed additional locks to the communal areas. The large glass doors in the homes and on the communal entrances look good but they don't look or feel secure. We've also found that the gates into the porches of the houses are too low, and people have climbed over them and stolen bikes. I feel the red brick exterior looks a bit stark but when the trees grow and people have plants around, it will soften the feel."
<B>How Coin Street is run</b>
Like all Coin Street's housing, the housing on site B is run as a co-operative. The homes are leased to Iroko Housing Co-operative, an independent registered social landlord, which lets the properties in turn to its tenant members.
The homes are let at affordable rents (from £76 a week including service charge for a one-bedroom flat to £123 per week for a five-bedroom house) to individuals and families in housing need who can show good reason to live in the area. Successful applicants must also be committed to taking an active part in the general running of the co-op.
Now the housing is occupied, full responsibility for its management has passed to the new co-op tenant members. All have completed an initial training programme of 11 three-hour sessions. An elected management committee of 15 people is responsible for the day-to-day running of the co-op between quarterly general meetings of all tenant members.