Planopedia

Infill development refers to the construction of buildings or other facilities on previously unused or underutilized land located within an existing urban–or otherwise developed–area. This type of development is meant to encourage density and accommodate environmentally sustainable urban growth by making use of existing utility and transportation infrastructure. Infill can mean development on vacant or formerly industrial land, such as former railyards, military bases, or parking lots, as well as the construction of additional units on existing residential or commercial lots, such as Accessory Dwelling Units (ADUs). Conversely, greenfield development happens in previously undeveloped areas, whether within a city or in exurban and rural areas.

The legal definition of infill development varies by jurisdiction, with local and state statutes and ordinances guiding each project. According to California law, an infill site has been "previously developed for qualified urban uses" or is adjacent to developed parcels. Qualified uses include "any residential, commercial, public institutional, transit or transportation passenger facility, or retail use, or any combination of those uses," as well as industrial uses.

Proponents of infill development claim that well-designed projects can reduce the distances people need to travel and their need for car trips and bring public transit within reach of more households, using existing infrastructure and resources to redevelop sites at a lower cost and with greater efficiency. Infill can revitalize communities and improve local amenities, housing options, and economic opportunities while countering the instinct for urban sprawl and keeping resources concentrated in smaller, more accessible areas. Infill supporters say it encourages more diverse and affordable housing options and brings housing closer to jobs, unlike suburban single-family developments which frequently exclude low-income households and place people farther from jobs and resources. Restraining development to existing urban areas with infill also helps protect wilderness, agricultural land, and other open spaces. 

Infill development often works in concert with urban growth boundaries, which designate the limits of growth for urban areas and protect undeveloped land on the periphery of the city. UGB policies often encourage and support infill development as a way to support growth without sprawl.

Conservation development, by contrast, is a set of policies usually implemented in suburban or exurban areas, where undeveloped land can be conserved alongside new development from the outset. 

Infill sites, such as those previously used for industrial purposes, sometimes pose challenges to developers that can include financing issues, inflexible land use regulations and zoning codes, expensive cleanup required at contaminated sites, and neighborhood opposition.

Despite concerns about higher initial costs, advocates of infill point to research from the Urban Land Institute showing that, in the long term, urban sprawl costs 40 to 400 percent more, or roughly $1 trillion per year in added costs to residents and governments. Sprawl also leads to a decline in quality of life factors such as air quality, commute length, and traffic congestion. 

Prominent examples of successful infill development projects include Portland's Belmont Dairy, Denver's Lowry, and Sacramento's McClellan Business Park, a former Air Force base that has been repurposed into a 16 million square foot business park.