The term 'Yellowbelt', coined by urban planner Gil Meslin, describes a large swath of land that is designated as 'Neighbourhoods' in the City of Toronto's Official Plan. The Yellowbelt could be defined by several residential zones and by-laws, but, as Sean Galbraith points out, the primary zone within the Yellowbelt is the Residential Detached (RD) zone - defined in the city of Toronto Zoning By-Law as areas that prevent higher-density development (through maximum height of 10 metres) and permit only detached residential housing:

10.20 Residential Detached Zone (RD) - In the RD zone, a dwelling unit is permitted in the following residential building types: (A) Detached House.

The term Yellowbelt is a reference to the Greenbelt (1.8 million acres of protected farmland and natural heritage in the GTHA) and its role as a restriction on new development for the purpose of preserving existing land uses and natural features. These 'stable' neighbourhoods restrict alteration of their streetscapes by limiting the types of housing that can be built, and therefore limit access to these areas to those that can't afford the high-value, low-density housing types. While there has been monumental high-density housing development in recent years, the European style walk-up apartment and mid-rise development, the so called 'missing middle', have been less popular with developers due to the lower number of units per developable hectare and lower profit margins. Therefore, developers may be left with limited options: either pay high prices for arterial fronting lots and develop towers to offset land acquisition costs or develop luxury single family homes in stable neighbourhoods.

City Councillor Gord Perks recently said the City has begun to address this policy issue through Official Plan Amendment (OPA) 320, which "amends the Healthy Neighbourhoods, Neighbourhoods and Apartment Neighbourhoods policies of the Official Plan..to protect and enhance existing neighbourhoods, allow limited infill on underutilized apartment sites in Apartment Neighbourhoods, and implement the City's Tower Renewal Program." While this OPA represents a significant change in the Official Plan, there are some limitations others have pointed out. Paul Demczak replied on twitter, "While policy 4.1.1 (from OPA 320) allows 4 Storey walkups, 4.1.5 states you cant introduce them into a context in which they are not currently represented," and then goes on to say, "This ensures predominantly detached and semi-detached neighborhoods will remain as such."