2.3 Retrofit and new design costs, benefits, and benefit-cost analysis

Standards Australia (2009) offers Australian Standard AS-3959, Construction of Buildings in Bushfire-Prone Areas. In locations subject to WUI fire, the standard requires non-combustible roofing. Requirements vary by hazard, as defined by a parameter called bushfire attack level. Sample requirements for relatively low hazard include:

• The bottom 400 mm of walls must be non-combustible or fire-resistant. • Windows can be fitted with non-combustible shutters, fine (< 2 mm) metal mesh screens, be made of safety glass, or be at least 400 mm above the bottom of the wall. • Exit doors can be protected by shutters, fine metal mesh screens, or be made of non-combustible material or fire-resistant timber. Garage doors have similar requirements. • Deck material must be non-combustible or fire-resistant timber if it is near windows (300 mm to 400 mm, depending on direction).

Requirements grow stricter at higher bushfire attack levels.

Intini et al. (2017) review WUI design standards and guidelines, along with construction, hazard, fire protection, and other issues, for Canada, California and the United States, Australia, New Zealand, Europe, France, and Italy. They do not address the quantification of benefits or costs.

2.3 Retrofit and new design costs, benefits, and benefit-cost analysis

The Multi-Hazard Mitigation Council (2019) presents benefit-cost analyses of compliance with the 2015 International Wildland-Urban Interface Code, both for new construction and for retrofitting existing buildings to comply with the code. It estimates retrofit costs as high as $70,000 and benefitcost ratios that can exceed 4:1 in high-hazard areas for retrofit and over 6:1 in high-hazard areas for new design. It estimates the costs for vegetation management to be $150 per year.

The Institute for Catastrophic Loss Reduction (2019) explains to insurers the incremental costs of satisfying the recommendations of the National WUI Guide. It explains the features of a fire-resistant house, such as non-combustible roof, cladding, and doors, explains recommendations for structure ignition zones, and estimates the costs of satisfying the National WUI Guide to various degrees. The authors estimate that to retrofit an existing home to satisfy the National WUI Guide would cost up to $15,000.

Hanscombe Ltd. and the Institute for Catastrophic Loss Reduction (2019) offer a cost-estimation spreadsheet tailored to various changes to make houses more resistant to wildfire.

RSMeans (2019a, b, and c) offers costs for residential, commercial, and industrial construction, including square foot costs for several categories of residential buildings, plus costs by RSMeans components, ASTM UNIFORMAT II assembly, and the classification system. RSMeans (2019d) provides costs for residential repair and remodelling tasks, such as demolishing and replacing exterior siding, replacing windows and doors, and other tasks likely to be relevant to retrofit existing dwellings. All four documents include location adjustment factors to account for costs in Canada.

Headwaters Economics (2018), in partnership with the Insurance Institute for Business and Home Safety (IBHS), finds that a new home can be built to wildfire-resistant codes for approximately the same cost as a typical home. The authors estimate the typical construction cost of a three-bedroom, single-storey, 2,500-square foot single-family dwelling in Park County, Montana, and then estimate the cost with wildfire-resistant detailing. They estimate the total construction cost to be $525,000,