Can you add a basement apartment in Westmount?

The rules, in short

• One accessory dwelling unit only, in a detached or semi-detached single-family house; not allowed in an accessory building. (art. 6.16.2.1 (By-law 1657))
• At most 35% of the floor area (basement included, garage excluded) and at least 51 m², with a bathroom, kitchen, living room and bedroom. (art. 6.16.2.2 (By-law 1657))
• No new door on the front façade; a new outside staircase or balcony only on the sides or at the rear. (art. 6.16.2.3 (By-law 1657))
• No additional parking space is required. (art. 6.16.2.4 (By-law 1657))
• Same water, sewer and electrical service and the same meter as the main dwelling; a separate address assigned by the City. (art. 6.16.2.5 (By-law 1657))

Summary by Radar Foncier, in our own words: the official French text of the by-law, linked above, prevails.

Applying in Westmount

• Urban planning department: 514 989-5219
• Permits on the municipality's website