Mixed-Use Retrofit: Exit Stair or Exterior Passage?
A three-storey building is being converted. The ground floor (about 420 m2) is becoming a restaurant; floors two and three (about 300 m2 each) remain offices. The existing single interior exit stair is 1 050 mm clear width and is not enclosed at the ground floor, opening directly into the restaurant seating area near the kitchen. The building is of combustible construction with a permitted floor area well above the actual area, and the owner wants to keep one stair and add a second means of egress through an existing exterior door at the rear of the restaurant that leads to a fenced service yard, then a gate to the street. Travel distance from the far corner of the third floor to the stair door is 33 m; from the far corner of the restaurant to the rear exterior door is 28 m. The designer proposes keeping the open stair and relying on the rear yard door as the second exit.
1. Evaluate whether the proposed two-path arrangement satisfies the exit requirements, identifying the controlling issues for each floor.
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The proposal fails on at least two counts and must be redesigned.
Stair enclosure. A stair serving floors two and three must be separated from the ground-floor occupancy. Because the ground floor is a restaurant (assembly-type use) and the stair opens into it, the stair must be enclosed in a fire separation with a rated closure at the stair door. The open connection is a vertical shaft connecting an assembly area to office floors above and is not acceptable regardless of the second path.
Two means of egress. Each floor needs two independent means of egress. The rear yard door is not a compliant exit for the upper floors because the path from the stair through the restaurant, out the rear door, across a fenced yard, and through a gate is not a protected continuous path and the yard is not a public thoroughfare. The gate and fence make the route unreliable and it passes through the restaurant's hazard area (near the kitchen).
Travel distance. 33 m to the single stair on floor three exceeds what one exit can serve; with two exits the limit is more generous, but the second path must be a real exit. 28 m in the restaurant to a single rear door is also questionable for an assembly occupancy with cooking hazards.
Width. 1 050 mm is adequate for the occupant load of the upper floors only if it is the sole stair for a small population, but once the restaurant's occupants share it, width and the convergence at the bottom become a problem.
Fix. Enclose the stair with a rated separation and self-closing door, provide a second enclosed exit from the upper floors (interior scissor stair or exterior stair), and treat the rear yard route only as a supplementary path, not as a required exit.
2. The owner argues that because the building is combustible construction and under the permitted area, the open stair is grandfathered. How should this be assessed?
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Area compliance is irrelevant to stair enclosure. Being under the permitted building area addresses building size limits, not the requirement to separate an exit from the occupancies it serves. A stair that is part of the means of egress must be enclosed where it connects floors of different occupancy or where the connected space is a hazard; opening a stair into a restaurant seating area beside a kitchen introduces smoke and fire exposure directly into the only vertical egress path.
Grandfathering applies to legally existing conditions that complied when built and are not being made worse. A change of occupancy of the ground floor to a restaurant increases hazard, so the alteration triggers upgrading of the affected elements. The correct position: the change of use makes the open stair non-compliant, and the alteration must enclose it. If full enclosure is impractical, an alternative solution under the objectives and functional statements could be considered, but it would need to demonstrate equivalent protection (e.g., a rated separation, sprinkler protection, and a smoke-tight vestibule) - not simply a claim of existing condition.