Which numbers count?

What is the point of density? It has become a staple of our housing policy, but is it a useful tool? The argument that we need to optimise the use of accessible land is compelling: it should help us to provide the buildings and spaces we need in our daily lives in a sensible, efficient and potentially sustainable way, rather than scattering them around like fairy dust.

But can density in itself get to the bottom of what compact development and long-term planning are about? Probably not. It is just a number. The maths is pretty basic, but can only be as good as the figures used. And it is here that density starts to become mystifying.

If we ask how many homes there are on a piece of land, we get a dwellings per hectare figure. If we ask how many rooms, we get another figure, and if we ask about floorspace, we might be given a ratio, such as 1: 3, to confuse us further. All might be relevant, depending on what we want to know, but figures can be hard to compare or visualise. And the same buildings could produce different figures depending on the size of units and the amount of non-residential floorspace.

Surely, what is important is how many people are going to be living on the site. That is a hard number to calculate but it is a real measure of housing provision and an indication of the services an area will require.

PPG3 excludes most of the places needed to make a group of dwellings into a community of homes

Then there is the measurement of the site itself. PPG3 defines a net site area, excluding most of the places and services needed to make a group of dwellings into a community of homes. If we were to stitch together a number of such net sites it would be most unlikely we would end up with a liveable neighbourhood, as the land needed for schools, shops, restaurants, parks, playgrounds, workspaces, train lines, water space and so on would be squeezed out.

If density policy is meant to secure optimum use of resources and create places people want to live in, then a more sophisticated technique is required than the net site or dwelling per hectare minimums. We need to understand how the homes we build, and where we build them, affects the number of people in a place and the geography of their daily lives.

We wish to build homes in a way that makes the best use of the services and facilities existing places can offer, but we don’t seem to think enough about how the character, transport opportunities and demography of that place can help us understand its optimum density. If we can start to understand the relationship between neighbourhood activity and housing density, maybe we will start to make sure we can make the best use of all types of resources, including existing buildings, neighbourhoods, public services and, of course, people.