If you measure in Units per Hectare (UPH) you will have net and gross densities. These two maps illustrate the difference. The first one measures the density in UPH of two-storey houses along the River Dodder. This gives a “gross” measure of 155 UPH. The second map measures a wider area, so that it includes open space, roads, rivers, linear parks, etc. This gives a “net” measure of 38 UPH and includes localised developments ranging from 25 to 120 UPH “gross”.
We can see that using Units per Hectare can be quite misleading. When we add into the equation occupancy rates, the figures get even more variable. At present, the average occupancy of a home in Dublin is approximately 2.4 people per unit. If that figure drops, we need more homes. If that figure increases, we need fewer homes.
Population and demographic trends also crucially impact how we measure use and need.
The dwelling unit type also affects density. For example, one two-bedroom home might have the same population density as two one-bedroom homes. So, the type of housing also matters in how we measure and plan density.
Taking all these and other factors into account, the best measure of density is population density – the number of people we need in a given area to support sustainable communities. This is the basis of the “15-Minute City”. There appears to be general consensus that a population of 25,000 aligns with the “Sustainable Neighbourhood” put forward by many and the 2002 Goodbody report that that figure was needed to sustain public transport.
The 15-Minute City
Walking, the 15-Minute City means a diameter of 1.25 kilometres. This gives an area of 5 square kilometres. With the “sustainable” figure of 25,000 inhabitants, this gives us an “ideal” density of 5,000 people per square kilometre (ppkm2). We can see from the 2016 CSO statistics that Dublin City has a density of 4,800 ppkm2, so our 2016 density was approaching a sustainable level.
For Dublin City and Suburbs, the 2016 density was 3,700 ppkm2, some way off the ideal. If we could raise this figure to 5,000 ppkm2, we could accommodate a further population of over 400,000.
For the RSES projections for the population of Dublin City to increase by 100,000, we will need to increase the density by 860 ppkm2, giving an overall density of 5,660 ppkm2. It would, in our opinion, be wise to plan for 6,500 ppkm2, to allow for further compact growth, or more open space, if the growth does not materialise.
In translating this data into how many new homes we need in the city, we should take account of demographic changes, which will impact on the occupancy of the existing and new building stock.
All of this indicates that densities should range from 65 to 100 UPH – and higher where appropriate in the context of a specific site’s size and context - whether that be through the re-use of existing buildings, the development on underused infill sites, the regeneration of existing neighbourhoods and the development – where appropriate - of serviced greenfield sites. This could be achieved on the city’s zoned lands, as well as the surrounding suburbs.
A key issue is defining the appropriate built form for any particular location. The lower street-based models – from three to six storeys – balance density with streetscape quality, and open space with residential amenity. Being more economical to build and maintain, it is therefore more affordable. Point blocks make for generally less-successful streets and public spaces, they create wind and microclimate issues, and their internal corridors are less conducive to the development of social capital. It is exponentially more expensive to build and maintain, and therefore outside the rental or purchase range of most salary earners.
The recently published Housing Design Handbook by David Levitt and Jo McCafferty is an excellent source of information on both residential densities and best- practice exemplars. The case studies it contains indicate that net densities of 65 dwellings per hectare can be achieved in three-bedroom terraced townhouses, and at two and three storey heights.
Bristol - a city with a population of 693,552 and an overall density of 3,900 ppkm2 - permits densities of 50 dwellings per hectare only where it is essential to safeguard the character and special interest of a historic location. Elsewhere, the optimum densities recommended in its Urban Living SPD (Supplementary Planning Document) range from 100 UPH in urban areas to 200 UPH in the city centre. By comparison, in Planning for Less Car Use, Friends of the Earth recommend densities of around 100 UPH, and higher in city centres - accompanied by ‘clear design guidance to ensure that housing quality, character, mix and sense of place are protected and enhanced.’
What the long-term impact of the Covid-19 Pandemic will have on the way that we live and where we live is yet to be analysed. However, it may have a major impact on the way that we use our cities.