Rendering by LGA Architectural Partners

The Ulster Condos is a five-unit missing middle development currently under construction in the Harbord Village neighbourhood of Toronto, designed by LGA Architectural Partners. The project converts the corner lot from an existing two-storey single-family dwelling and rear garage to a three-storey multiplex and laneway suite, an excellent example of gentle densification within existing low-rise neighbourhoods.

Each of the units of the multiplex are accessed separately with individual entrances. The upper units are organized back-to-back with bedrooms on the second floor and living spaces on the third floor. These two upper units require a total of four stairs, consuming a significant proportion of the usable floor area and restricting the units to a stacked layout.

This design study explores an alternate spatial arrangements for these upper units if a single egress where allowed, such that both units are accessed by a single common stair.

Proposed and Alternative Floor Plans
Note: no change to basement and laneway unit plans.

Floor Area Calculations
The single egress design results in a reduced total leasable area because the common stair cannot be applied to either unit, however the usable area is increased as the total area consumed by stairs is drastically reduced. This results in additional usable area for unit 1 and most significantly, an additional bedroom for unit 3.

Note: No changes were made to the total building footprint and balcony areas for each unit.

It is important to recognize that the egress requirements of Part 3 (Fire Protection, Occupant Safety and Accessibility) of the Ontario Building Code do not apply to this project. As this building is of Group D residential occupancy and of “three storeys or less in building height and having a building area not exceeding 600m2”, this project falls within Part 9 (Housing and Small Buildings) of the code. Part 9 allows for residential buildings of up to three storeys in height with a single exit stair if each dwelling unit has its own direct exit to grade (such as townhouses and stacked townhouses).

Part 9 also establishes distinct rules for the allowable rise and run of treads in exit stairs, depending on whether they serve a single dwelling unit (private stair) or serve more than one dwelling unit (public stair). The diagram below compares the steep rise and run allowed for Part 9 private stairs to the slope of Part 9 public stairs as well as exit stairs in Part 3 of the code. This reveals the bizarre reality that the 2012 OBC requires a more stringent 280mm run for public stairs in Part 9, but only 255mm for exit stairs in the more onerous Part 3. 

Update: As of January 1, 2022 the 2012 OBC Part 3 exit stair dimension of 255mm min. run and 200mm max. rise has been decreased to a 280mm min. run and 180mm max. rise, now aligned with the more stringent Part 9 requirement for public stairs. The below diagram and text is from 2021.

In the case of the Ulster Condos, adherence to the requirements for a public stair serving two dwelling units increases the minimum length from 3570mm to 5320mm, a drastic increase of about 50% compared to a private stair serving one dwelling unit.


Construction Documents provided by LGA.

Nugent, Michael. (2021). Post[Sub]Urban Complex. CRIEM (Center for Interdisciplinary Research in Montreal Studies). Source of Context 3D Assets in Massing Diagram.

Ontario Building Code.