Analysis – How Ottawa can Cut Carbon Emissions and Deliver Housing with Fill it First

Ottawa’s Growth Challenge

Ottawa has pledged to build 151,000 new homes by 2031. How and where these homes are built will determine whether growth makes life more affordable and climate-friendly — or locks in higher costs, congestion, and climate emissions. The City of Ottawa faces a choice: continue the traditional pattern of greenfield development through outward expansion, or “fill it first” by adding gentle density within existing residential neighbourhoods closer to jobs, schools, transit and other amenities.

The Housing Case for “Filling it First”

Ottawa has a significant opportunity to house many more people on in existing urban and suburban neighbourhoods, prioritizing infill development before expanding to new car-dependent subdivisions. Hence: “Fill it First”.

The city’s new zoning bylaws will make it easier to add secondary suites to a single-family home or back yard, convert single homes into multiplexes, or build small apartment buildings along major streets. In addition to providing alternatives to urban sprawl, these neighbourhood housing options can mitigate an over-reliance on large-scale apartment and condominium projects that typically offer smaller units that may not align with the needs and preferences of many households.

The Climate Case for “Filling it First”

Not only does gentle density infill provide a diversity of homes within residential urban and suburban neighbourhoods, but modelling commissioned by OCAF finds that this approach is the most effective pathway for Ottawa to reduce climate emissions from new housing growth. Adding secondary suites within existing neighbourhoods can cut greater amounts of carbon emissions than adding the same number of homes via urban sprawl or through larger, denser apartment buildings.

The Analysis: Four Growth Pathways

The study compared four growth scenarios — each adding 15,000 new housing units in Ottawa, compared to the average trend of 5,500 new units per year:

• Pathway 1: Single detached development primarily on the urban edge, in areas like Kanata-Stittsville, Barrhaven, Riverside South and Orleans
  • Reference case
• Pathway 2: Hub apartments defined as new apartment buildings in central and suburban hubs for employment or post-secondary education
  • 35% GHG savings compared to reference case
• Pathway 3: Distributed suites a scenario in which secondary suite renovations are distributed equally across Ottawa neighbourhoods
  • 41% GHG savings compared to reference case
• Pathway 4: Hub suites consisting of secondary suite renovations in single-family detached homes in central and suburban hubs for employment or post-secondary education
  • 50% GHG savings compared to reference case

As Figure 1 illustrates, Fill it First strategies deliver significant carbon reduction potential, with secondary suites added to existing houses and yards offering the greatest opportunity for emissions reductions. Adding housing via multi-unit residential buildings in urban and suburban hubs offer comparable emission reductions associated with transportation as hub secondary suites. But emissions attributed to homes themselves—both operational (energy use) and embodied carbon (materials to build)—are lower for secondary suites.

Transportation, Location and Carbon Savings

Buildings and transportation make up roughly 90% of Ottawa’s emissions. The analysis examines the locations for new housing and draws a clear connection between how land use planning influences transportation, household transportation patterns and associated transportation emissions. Land use planning and the location of where we build housing influences the types and sizes of homes and associated emissions and embodied carbon from housing.

Figure 1 illustrates that the largest emissions associated with new housing in Ottawa are attributable to land use planning and resulting tailpipe pollution from transportation. The most important planning determinant of household car ownership rates and driving distance — and thus transportation congestion and carbon — is location. According to sources cited in the study, proximity to an urban region’s central business district has the biggest impact, followed by secondary job hubs and services, notably grocery stores. Daily commute and long travel distances from suburban or exurban, single-family homes can double or triple a household’s carbon footprint (Figure 2).

Examining operational emissions and embodied carbon

The modelling compared emissions across the four land use strategies in two different buckets: embodied emissions from building a home or manufacturing a vehicle, and operational emissions from driving a vehicle or heating and cooling a home. It found that single detached homes would produce almost twice as many operational emissions as secondary suites.

The main factors determining those results included:

• The square footage of a single-family home—for the most part, larger homes mean higher carbon footprints;
• Shared walls in multi-unit buildings, large or small, that can reduce the energy use and carbon emissions from individual units;
• Small multi-family developments and compact home designs that can reduce construction emissions and embodied carbon for each home.

Housing Typology Emissions and Opportunity

The study also analyzed the carbon savings potential associated with housing typologies. Most of Ottawa’s housing stock is single detached homes, which account for over 40% of dwellings. Yet only about 2% of these homes have secondary suites (See Figure 3).

The modelling also found that half of Ottawa’s detached homes are occupied by just one or two people, while the fastest-growing household type is singles living alone. This represents an enormous housing opportunity to add units to underutilized dwellings and the lots, via secondary suites and conversions to multiplexes.