The residential perimeter block: principles, problems and particularities

The perimeter block remain for us the most useful and valid starting point for the residential components of our masterplans. There are of course myriad ways of composing urban form, each with their own merits or disadvantages in any given situation. But even when proposing alternative forms, we regard the perimeter block as a model against which these can be developed, tested and understood. Why has this model remained so compelling, and what are its limits?

A perimeter block is an urban form that concentrates the development of a city block along its outermost - or public - edges. A number of important consequences flow from this. Perhaps the most significant is that it reinforces and makes clear the underlying pattern of privately owned plots, streets and public spaces that constitute the city. But it is not simply a question of clarity. By strongly defining their edges, perimeter blocks enable the streets and squares of a city to be thought of as positive spaces, as urban rooms. And by placing commercial uses in direct contact with this public realm, the perimeter block provides the best possible opportunity for contact and exchange.

But external space in cities is not always public. Because the depths of buildings are defined by the need for adequate daylight, the perimeter plan naturally produces an open space at its centre. This space has no obligation to be public and - in many cases is best not to be so. In traditional perimeter blocks - those containing a number of different land ownerships - this space is usually allocated to private gardens, yards and ancillary buildings. In schemes where a single development encompasses an entire urban block, this central space is more often given over to a variety of communal functions: play spaces, gardens, cycle parking. In either case, the perimeter block typology promotes an explicit distinction between the private or communal realm and the public realm. This absence of ambiguity is important. Each type of use has its oen 'rules' of behaviour and, importantly, its own maintenance regimes. Other typologies that produce more ambiguous patterns can be hard to make work. The forlorn and unclaimed land that surrounds many stand-alone post-war housing blocks are obvious examples of ill-defined communal space.

The structure of towns and cities tends to be hierarchical. Just as there are grand squares, high streets and parks, so there are back streets, side streets, narrow lanes and alleys. Each urban block necessarily has to form its own set of relationships within this wider urban matrix - its own fronts, backs and sides, its own particular aspects and prospects. Because they are usually large in plan but still able to be conceived as a series of discrete pieces, perimeter blocks allow a certain looseness of composition, the perimeter block is a type that is fundamentally well disposed to respond to its place in the city fabric. Each side of a block can adopt a form, scale and character suited to the street or space that it addresses. No matter how secondary the frontage of a perimeter block may be, it always defines a street frontage.

Because the perimeter block makes a clear distinction between these fronts and their corresponding backs, their rear elevations - relieved of any obligation to play a wider role within the city - are able to develop a more ad hoc character in response to specific uses of the nature of the central courtyard space. While this distinction is more evident in traditional mixed ownership blocks - in the tenements of nineteenth-century Edinburgh for example - and even more so where these contain a mix of uses, this conceptual framework can still provide opportunities for architectural expression within single ownership blocks. This fundamental flexibility also enables a variety of different building uses to be incorporated within a normative urban pattern. Large scale/deep plan uses such as car parking - whether below ground or beneath a podium - is the most common of these, but large retail units, energy centres, schools, health facilities and even offices can similarly be incorporated.

Lastly, the perimeter block also offers a solution to the efficient use of land, a point made by Leslie Martin in hist 1972 essay "The grid as a generator: exploration of a new theoretical framework for the use of land in cities". Martin noted that density need to not be equated with tall buildings, pointing out that for a given area of land, a courtyard form would achieve the same density as a tower form of three times the height. While this argument ultimately led Martin to advocate the development of large mixed-use super blocks where the block interior is public, such as Patrick Hodgkinson's Brunswick Centre, 1965-1973, it also provided justification for projects such as the residential courtyard block at Clipstone Street designed by Michael Gold for Federick Macmanus and Partners, 1966-1971, and remains relevant to any reasonably sized perimeter development.

The efficient use of land and the maximisation of density have become increasingly important issues in contemporary urban planning. In London, according to the Greater London Authority, projected population rises mean as many as fifty thousand new homes are needed every year. It is perhaps unsurprising then that in the city's identified growth areas - typically on former industrial land on the edges of the city centre or in the Thames Gateway corridor - the residential densities proposed are significantly higher than those in the surrounding areas, with over four hundred dwellings per hectare being uncommon.

Alongside - and partly in reaction to - these strategies for growth, new planning policies and guidance have emerged that reflect a growing concern for the quality of newly built residential accommodation, including requirements for minimum internal floor areas, private amenity spaces and communal external space and, perhaps most importantly in the context of high density development, for daylight, sunlight, and natural ventilation levels within flats. These changes are overwhelmingly positive. Setting aside the complexities of housing affordability and stability of tenure, creating homes that are genuinely fit for purpose, that people like and therefore want to live in, is arguably the best way to encourage the kind of stability and social cohesion more traditionally found in neighbourhoods of houses.

In recent years this tension between increasing density and raising standards has prompted the development of a number of different types of perimeter block. At a height of between three and six-storeys perimeter blocks remain low enough to ensure adequate sunlight and daylight to both the courtyard and the blocks interior elevations. But as densities increase and building heights rise, this becomes impossible to achieve without manipulating the basic perimeter block form in some way. These manipulations can involve either breaking the block in plan, or stepping down in section, or both. Perhaps the most striking departure from the basic type is seen in the hybrid perimeter block and tower arrangement, which, in a rebuttal to Martin's argument, explores whether it might be possible to have both. These hybrid forms are characterised by a perimeter block from which one or more taller elements rise. At the corners they may either be free-standing or integrated into the form and architecture of the base block. A second type is the stepped block, here building heights rise and fall around the perimeter in response to orientation - either to admit southern light into the courtyard or prevent overshadowing on its northern edge - an approach which, if pursued to its logical extreme would invariably generate parallel rows or north-south orientated buildings, antithetical very often, to the urban condition in which they are located.

The tension therefore now pulls three ways - between quality and quantity - and also urban clarity. In reconciling density with quality, we need to be careful not to undermine the spatial clarity and continuity of the city fabric and to acknowledge the reciprocal nature of the relationship between the configuration of urban form and nature of the public domain.