Density Calculator

First published 17 September 2020, last updated 23 September 2020.

What is the density calculator?

The density calculator is essentially an interactive version of Martin and March's (1972) arrays of pavilions, streets and courts or Berghauser Pont and Haupt's diagram of how street widths and spacing affects the relative area of public to private land (2009, p.124) - but with the buildings added.

What does it do?

It takes two very common arrangements of buildings in cities and puts them side by side on an equally sized grid with a common set of sliders. The sliders allow you to adjust key dimensions that determine the urban form (block length/depth, street width, building depth and any set back between the buildings and the public highway).

It then displays on the bar chart the relative land use area split between the street (public highway), the gardens (areas of open private land), and the building footprints. It shows the gross split (i.e. including the public highway) in bold text, and the net split (i.e. excluding public highway) in light text. The proportion given over to the building is equivalent to the Ground Space Index (GSI) or, for the net numbers, plot coverage.

What does it show?

It shows that the relative land uses in these two extremely common urban forms have fundamentally different relationships with the street layout and it also shows that what might be considered 'optimising land use' (i.e. fitting more buildings in) may look different depending on whether your perspective is that of the city (responsible for all land including streets) or the land owner (only interested in private open land or buildings).

Key highlights

These are just a few key observations that can be made:

• In both cases reducing block depth increases gross building density. The area of developable land is reduced and the proportion of land given over to the street network is increased but 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 gross building density. A greater proportion of private land is transferred to public highway but the quantity of building as a proportion of the whole increases. In the case of row/terrace housing the reverse is true. Decreasing block length decreases gross building density. The increased proportion of land given over to the public highway network is not compensated for by extra building. The tighter street spacing appears to be a cost not only against the amount of private land but also the total building capacity.
• Increasing street width decreases gross building density but increases net building density. In many so-called 'sustainable' developments there may be pressure to increase the width of public highways to accommodate segregated cycle paths and encourage walking. The width of the highway may also be increased to accommodate 'green' features such as trees and areas of grass. In planning systems which prioritise net density measures, the effect may seem positive yet it may go un-noticed that it has actually driven down gross density which may be in direct conflict with other sustainable development aims.
• The impact of set-back differs in the two typologies. In terrace/row housing it has no impact on gross or net density, it simply serves to move the buildings backwards or forwards in their plots. In the courtyard/perimeter block typology, setting the building further back from the street decreases both gross and net building density driven by the size of the building that can be accommodated on a corner plot.

Why is this important?

Cities typically have to balance competing demands to optimise land use, increase residential capacity and improve permeability yet land use and transport planning are often treated entirely separately. If the differing relationships between building typologies and the street network in the various urban forms are not well understood it may be, for example, that aiming to optimise land use while only building single-family housing might automatically lead to long sprawling suburban streets, or a desire for generous street widths with green space and separated carriageways unintentionally makes a proposed residential development unviable due to the limited residential capacity that the street design unintentionally creates, or in planning systems with ambiguous distinctions between public and private space it may simply be impossible to make any useful consistent quantitative link between urban form and residential density/capacity at all.

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.