+64 223 152 592

How infill housing can work with existing infrastructure


Above: Gully-trap
porous paving solution Ian McComb Small Time Developments Limited
Above: A porous paving solution
Household size decline
"Infill housing is the insertion of additional housing units into an already-approved subdivision or neighborhood. They can be provided as additional units built on the same lot, by dividing existing homes into multiple units, or by creating new residential lots by further subdivision or lot line adjustments."


Few people would question that we need more infill housing as part of the solution to the affordable housing crisis.  However, one of the legitimate concerns for infrastructure suppliers when considering increasing the density of urban areas is the need to upgrade the existing services in order to meet the needs of a larger number of people within the same street or suburb.  Increasing the size of infrastructure in existing urban areas is hugely expensive compared to building in an open paddock due to the need to and carefully dig around existing services, make it safe and then reinstate the existing streetscape.

It would seem to make sense that extra dwellings = extra people = extra demand on infrastructure.  However, as I outline below, there are several reasons why this is not necessarily true.

Household size

Due to the ageing population, relationship breakups and less children per household, the number of people in households has been declining for decades.  Therefore, replacing five houses with 10 smaller ones could easily end up housing fewer people than the number who lived in the same area in fewer houses in the 1960s.  Graphs showing the trends over the past 40 years of increasing house size and decreasing household size are available here: https://www.stuff.co.nz/business/property/94686507/why-are-we-building-such-big-houses

Vehicle use

The increasing use of electric mobility devices has meant that many trips are not needing cars.  For example, Hamilton City Council has just reported “Over 7000 people enjoyed 13,000 rides on a Lime scooter on the weekend, travelling a whopping 25,000km across our city!” at: https://www.facebook.com/search/Over 7000 people enjoyed 13,000

Peak oil, ride share vehicles and other trends will seriously undermine current assumptions about levels of car use and ownership in the future.  This means most roads will not need to be widened to increase their capacity to carry more cars, and most housing developments will not need more than one car space per dwelling.


With the increasing availability and use of double glazing, better insulation, power saving light fittings and appliances (https://www.energywise.govt.nz/energy-labels/) and options to install solar power without incurring a significant upfront cost (e.g. https://www.solarcity.co.nz/) there is no reason why two modern houses need to use any more power than a single old one does.

Water Supply

Similarly, with the development of water saving appliances (https://www.mfe.govt.nz/fresh-water/we-all-have-role-play/choosing-water-efficient-products) and the trend towards having smaller gardens, the demand for water can be significantly lower for modern dwellings.  Replacing or fixing leaking pipes in older areas as part of the process of providing infill housing will also help to neutralise the impact of development.


Less water use inside households also reduces the amount of wastewater which needs to be piped to a treatment plant.  However, two other aspects that are often more significant here are inflow and infiltration (I&I).

Inflow occurs where a stormwater pipe is illegally connected to the sewer pipe or when the gully trap is too low and therefore collects surface water when it rains. (A gully trap is a basin in the ground which receives piped wastewater from your kitchen, bathroom and laundry before it is emptied into the sewer.)
Infiltration occurs when a pipe is broken, or the pipe material become porous due to age (allowing groundwater to pass into it), or a tree root or earth movement forces a gap at the joints of a pipe. The amount of water leaking into underground pipes increases when the groundwater level is high (due to heavy rainfall and/or the effects of sea level rise) and can become very significant in old pipe systems.

Both of these issues are usually relatively easy to fix once they have been identified.

Therefore, taking the opportunity to fix the I&I issues described above during the development of infill housing can easily allow for more dwellings, and reduce the amount of wastewater entering the pipe network which then has to be processed at a wastewater treatment plant.  This reduces costs associated with increasing the size of wastewater pipes and the capacity of treatment plants.  It also reduces the risk and volume of overflows of partially or untreated sewage, which can have cultural, health legal and environmental impacts.


Stormwater generation is often significantly increased by infill development.  This occurs when grassed or vegetated areas are replaced with buildings and concrete.  While some loss of green space is inevitable as a result of development, less absorption of rain water by the land needs to be compensated for through the use of methods such as detention tanks, which are now commonly required by councils at the time of development.  Detention tanks store water during rainfall events, and slowly release it to the stormwater system later, at a rate that can be contained within the stormwater pipes, be absorbed by low impact design treatment systems, and/or be discharged to rivers and streams without causing erosion or adding sediment to waterways.

Overseas, and to a minor extent in New Zealand, green roofs are reducing the runoff from new and existing buildings (see http://www.livingroofs.org.nz/).  Porous paving is another way to provide durable all weather driveways without generating excess stormwater.

Planning Rules

The Auckland Plan has provided for minor dwellings up to 65m2 in size to be built on existing properties and PrefabNZ has run the very successful SNUG design competition that led to many innovative designs that could serve this need.  http://www.prefabnz.com/Projects/Detail/snug-home

However, in order to allow infill development without overtaxing existing infrastructure, we believe that the above solutions need to be required for both the existing dwellings on a property and the new dwelling at the time of the development so that the net impact of the additional dwellings is very low, and hence manageable within existing infrastructure renewal and expansion budgets.  This change will require either central government action or individual District Plan changes by each Council.

Urgent action to reduce the housing crisis

Rule changes to facilitating quality infill housing can significantly reduce the housing crisis, but will our Councils make it happen in a reasonable time frame?

I believe that in medium and high growth areas, it is too important to combine with a larger District Plan change and needs to be progressed as a dedicated priority project.

As it's Council election time, which candidates will endorse this approach?

Copyright © Small Time Developments Limited

Privacy Policy
phone-handset linkedin facebook pinterest youtube rss twitter instagram facebook-blank rss-blank linkedin-blank pinterest youtube twitter instagram