NOTE: The first version of the street layout and density calculator is available at this webpage. This notebook contains the source code.
Residential density appears to be a straightforward concept: some measure of housing (number of dwellings, building footprint, total floor area etc.) expressed as a ratio relative to land area. Increasing the size/volume of buildings as a mechanism for increasing density is well understood. Adding storeys or replacing small buildings with new ones with a bigger footprint are typical responses in the drive towards densification.
However, cities have a responsibility to manage the total area of land available to them. In simple terms this means the public spaces (including the highway) as well as the private land and the buildings built upon it. What is perhaps less well understood, particularly in cities that monitor density against plot or site area, is the role that the street network has to play in determining the relationship between urban form and overall density.
When ‘optimising land use’ (often a euphamism for increasing density) there is an interrelationship between the desired urban form, the street network and the resultant density. The relative permanence of the layout of the street network may explain the lack of focus on its role however, large scale urban regeneration projects and urban extensions are opportunities to revisit it.
This interactive web page is a first attempt at illustrating the relationship between the street network, urban form and density in two common types of urban form, the perimeter block and terrace/row housing. It demonstrates how two superficially similar building arrangements (abutting buildings aligned along the street) have fundamentally different relationships with the street network.
Some key points to note:
- In both cases reducing block depth increases building density. Even though the area of developable land is reduced and the proportion of land given over to the street network is increased the tighter spacing between buildings (at the expense of garden space) increases gross density.
- In the case of the courtyard/perimeter block, reducing block length increases overall building density. Again a greater proportion of private land is transferred to public highway but again the quantity of building as a proportion of the whole increases. This is due the buildings making full use of the extra street frontage generated.
- In the case of row/terrace housing the reverse is true. Reducing block length decreases overall building density. In this case the extra frontage generated by giving land over to the public highway network is not compensated for by any extra building. The tighter street spacing appears to be a cost not only against the amount of private land but also the total quantity of building.
- Street width: in many so-called ‘sustainable’ developments there is pressure to increase the width of public highways to accommodate segregated cycle paths and to encourage walking. The width of the highway may also be increased to accommodate ‘green’ features such as trees and areas of grass. Again, particularly in systems which monitor density against plot area, it can go un-noticed that, if back gardens are not reduced to compensate, this will have the effect of driving down overall density which may be in direct conflict with the stated aims of the sustainable development.
- The impact of set-back differs in the two typologies. In terrace/row housing it has no impact on overall density, it simply serves to move the the buildings forwards or backwards in their plots. In the courtyard/perimeter block typology, due to the uneven relationship between the front and back garden at corner plots, setting the building further back from the street decreases overall building density.
Increasing overall/gross density at a city scale is a simple matter of increasing the quantity of residential building relative to total land area. However it is worth remembering that at the city scale, the layout of the street network itself will impact both overall density and total capacity and that this may go unnoticed particularly if density is only monitored against the site or plot.
References
- Berghauser Pont, M., & Haupt, P. (2009). Space, Density and Urban Form. https://doi.org/9789052693750
- Martin, L., & March, L. (1972). Urban Space and Structures (L. Martin & L. March, eds.). London: Cambridge at the University Press.
- Unwin, R. (1909). Town Planning in Practice: An Introduction to the Art of Designing Cities and Suburbs. London: T. Fisher Unwin.