Assessment of Zero Energy Prefabricated Accessory Dwelling Units in Support of New Zealand Housing Needs

Problem statement: The demographics of New Zealand are changing and the country is getting older. The literature shows the housing needs of older people are different, and that many would prefer to stay in their existing neighbourhood and age in place. In addition to the shortage of houses, low residential densities, the fast growth of land and house costs over income, and an aging population are all current issues in New Zealand housing. Projections also show the shortage of energy resources and environmental pollution will affect the future of housing, as the housing industry is responsible for over one-third of global energy use and CO₂ emissions.  Aim of the research: This thesis aims to design and evaluate the practicality and efficiency of a prefabricated Accessory Dwelling Units (ADU) as a partial response to New Zealand housing needs. Prefabrication was selected as the method of construction as it has been claimed to be more efficient in terms of energy and material use, as well as leading to a shorter construction time and lower environmental impacts. The use of ADUs could increase residential density and add to the housing stock by using existing developed land and infrastructure. ADUs also offer smaller houses within the existing social context, which the literature suggests is the main housing requirement of older people. As a result, the ADU in this research was designed to suit older residents, whilst recognising that it would also be suitable for small households of all ages.  Methodology: This research used design as a tool with which to explore the potential of prefabricated ADUs as a contribution to New Zealand housing needs. The design was then tested against housing needs by using Life Cycle Analysis (LCA). This part of the research was conducted in the three phases of life-cycle energy, life-cycle cost, and life cycle CO₂ emissions of the ADU as designed.  Results: While the transport limitations pushed the design to be narrow (the maximum allowed load width was 2.55m), the Lifemark accessibility standard asked for doors, corridors, and spaces wide enough to ease the movement of disabled people. Despite these difficulties, it was possible to design an ADU which could be manufactured in New Zealand and transported anywhere in the country without any need for over-dimension load permissions. However, the analysis in the last phase, looking at the ADU during its life span, showed there was no substantial difference between its performance and that traditionally made houses. The results of the analysis suggest that, despite the importance of the construction method, the building energy efficiency, CO₂ emissions and cost, the environmental impact of a house is more dependent on the choice of materials than the method of making it.  Future research: The results suggest the importance of further investigation into the choice of materials used to make residential buildings and the effect such choices have on life-cycle impact. There is also a need to seek feedback on the ADU as designed from both potential users and potential manufacturers.