This is an important chart from a recent study commissioned by Greater Wellington, New Zealand. The study looks at the cost benefits of urban intensification and the above chart shows the relationship been density and infrastructure costs. For this study, they specifically looked at the costs that local governments face in providing road, public bus transport, and "three-waters infrastructure." I hadn't heard this latter term before, but it refers to drinking water, wastewater, and stormwater.

What they obviously found was that there are real economies to higher densities. More density lowers the per dwelling cost of delivering infrastructure. In the case of three-waters infrastructure, it doesn't even really matter if you're proximate to reservoirs or treatment plants. The bulk of the cost lies in the local connection pipes. So what matters most is how many dwellings you can service off of the main lines -- even if these lines need to be upsized.

The goal of this study is to enable more support for smart growth within the Wellington region:

Regional councillor Thomas Nash says the report should give councils confidence to press on with plans that support compact mixed-use development in and around city centres and connected by high quality public transport. 

“Regional growth needs to be smart growth. This report clearly shows that the best bang for our buck is to focus on upgrading existing water, public transport and local roading infrastructure so that we can build better quality, compact residential form, with improved amenities within our cities and towns,” Cr Nash says.

Of course, this doesn't just apply to Wellington. Every city should read the study.