Basement Additional Residential Unit Structural Design – Markham
This project involved the structural design and code coordination for a new additional residential unit within the basement of an existing two-storey dwelling. The scope focused on enabling a compliant second unit while maintaining the integrity of the existing structure.
The work was developed in alignment with Ontario Building Code requirements, addressing fire separations, ceiling height constraints, spatial separation, and life safety provisions specific to basement ARUs.
Location: Markham, Ontario
Completion: January 2025
Sector: Residential
1. The Challenge
The existing basement was to be converted into a self-contained additional residential unit within an occupied dwelling.
Existing Structural Conditions:
The basement slab, foundation walls, steel beam, and posts were to remain, requiring the new design to work around fixed structural elements without modification.
Code Compliance Constraints:
Minimum ceiling heights, fire separations between units, and spatial separation limits for glazing had to be met under OBC provisions for secondary suites.
Life Safety Integration:
Fire-rated assemblies, interconnected smoke and CO alarms, and protected egress needed to be integrated without altering the existing superstructure.
2. The Engineering Solution
A structural and coordination package was developed to support permit approval for the basement unit under OBC Division B requirements.
a) Structural Layout Coordination:
The proposed basement layout was coordinated with existing beams, posts, and foundation walls, maintaining all primary structural elements as-is.
b) Fire Separation Design:
Fire-rated wall, floor, and ceiling assemblies were detailed to achieve required fire-resistance ratings between units and common areas.
c) Spatial and Opening Compliance:
Window sizes, lintels, and opening limits were reviewed to satisfy spatial separation and glazing percentage limits per OBC Tables 9.10.14.4.
3. The Outcome
The project resulted in a coordinated, permit-ready basement secondary unit design accepted by the City of Markham. The solution enabled a code-compliant additional dwelling unit while preserving the existing structure, addressing fire safety, spatial separation, and minimum height requirements in accordance with OBC standards.