Sustainable
Density
Ireland has many developments that meet recognised standards for socioeconomic sustainability and density. Here we outline what this means and show some examples. In these examples, we use the term "sustainable" to refer to density, rather than in terms of climate change, bio-diversity and carbon use.
WHAT IS DENSITY?
At its simplest, sustainable density could be described as the number of people in a given land area that can sustain the services that they require to maintain a healthy life that has the least impact on the earth, climate and biodiversity.
The challenge for us is how to address a growing population and a changing demographic.
In terms of assessing sustainable density, population is the only empiric measure of the density of our neighbourhoods, towns or cities. Population is the ultimate measure of vitality, vibrancy and viability.
We generally use UPH – Units per Hectare (also described as units/hectare). The correlation between UPH and population density can be very variable. For example 100 one-bedroom apartments could have the same population as 50 two-bedroom apartments, but at nearly half the built volume. These things matter. Similarly, if half of the apartments are unoccupied (as in some recent developments), the population is halved.
Units per hectare is a coarse instrument – people per square kilometre is a better measure, but is difficult to plan for. We have tried bed-spaces per hectare in the past but that method too is not truly predictive of population density.
WHAT IS A SUSTAINABLE MODEL?
The current model that is promoted is the 15-minute city. This means that all facilities for living should be within a 15 minute walking distance, extended further by bicycle use. In practice this means that a 1.25 km radius would result in an area of 5 square kilometres. The core population needed to sustain social, physical and environmental infrastructure is considered to be 25,000 people. This gives a required population density of 5,000 people per square kilometre, or 50 per hectare. Emerging research indicates that, irrespective of occupancy and dwelling type/size, the gross figures for population tend to be equivalent to the nett units per hectare (units/hectare) on a given site.
This indicates that, in general, a dwelling density of 50 units/hectare will result in the 5,000 people per square kilometre that sustains the 15 minute city. Given that the existing density in some areas might be lower and that most areas could assimilate the predicted population growth, a nett density of 65 to 100 units/hectare could build the housing stock that we may need, without undermining the existing quality or community.
NATIONAL PLANNING POLICY
Compact Growth: The National Planning Framework promotes sustainable growth of compact cities, towns and villages to add value and create more attractive places in which people can live and work. Its long term vision for Ireland's housing future "...Aims to balance the provision of good quality housing that meets the needs of a diverse population, in a way that makes our cities, towns, villages and rural areas good places to live now and in the future."
Unlike the arguments used to impose excessive densities in inappropriate places, National Policy (and all of the other guidance, mandatory or otherwise) is very clear that appropriate scale is a core value.
HOW DO WE INTEGRATE HIGHER DENSITIES SUSTAINABLY?
If we are to succeed in densifying. We should focus less on building height and more on how we can achieve compact growth, maintaining the quality and character of our neighbourhoods and ensuring that we protect our built heritage.
The claim that increased building height is the answer to densification is unsubstantiated. None of the European cities that we aim to copy are high. There is extensive research to prove that height does not automatically increase urban density.
Perhaps the best method of controlling the scale, quality and density is through the tried and tested means of site coverage ratios, plot ratios, private and public open space criteria and internal space standards. In these times of centralised planning, these are perhaps the only tools that the Councils can use to influence the form, density and quality of our future.
For example, the current population of Dublin city (2016) is 554,554 – a density of 4,800 people per square kilometre. For the pre-consultation Draft Dublin City Development Plan figure of 70,000 more people, this means an increase of 12.5%, resulting in a density of 5,400 p/sq.km. For the RSES projection to 2013 of 100,000, this means 850 additional people per square kilometre.
This is, spatially, an easily achievable number. The number of ‘units’ this requires depends on the occupancy. At present Dublin has an average household size of 2.48, lower than the national average. So, for 600 more people/sq.km., we might need 250 more ‘units’ /sq.km; at 850, perhaps 340 ‘units’ /sq.km. How many ‘units’ per hectare this equates to individual sites and areas is difficult to establish, but the general rule might be that the increase in density should be proportionate to the existing density.
EXAMPLES OF SUSTAINABLE DENSITIES OF DUBLIN'S BUILT ENVIRONMENT
Dublin is full of examples that demonstrate that successful housing can be integrated with the locality and the community. You can find many of these examples in our Sustainable Density Developments map. Click on the markers and images for more information on each development.
All the developments are at densities ranging from 40 to 250 units/hectare. The highest densities are in the centre of Dublin, within the canals. Those outside the canal ring and in suburban areas are generally at a lower density.
All these developments pre-date the Strategic Housing Development legislation and the Section 28 Mandatory Ministerial Guidelines. We have yet to see any new supply from the SHD process.