What is a party wall agreement and do I need one for a semi-detached home in Ontario?

A party wall agreement is a document between the owners of two adjoining properties, most often semi-detached homes or rowhouses, that sets out each owner's rights and responsibilities for the wall shared between them, since that wall structurally supports and serves both units at once. It typically addresses repair and maintenance responsibilities, cost-sharing, requirements for advance notice before either owner alters or renovates near the wall, and how disputes get resolved.

Ontario doesn't have a general province-wide statute requiring semi-detached owners to have a party wall agreement, so whether one exists depends entirely on whether the original builder, developer, or a later owner put one in place and registered it on title. Where no such agreement exists, owners still have rights and obligations toward each other, but they come from general common-law principles around shared structures rather than a specific written framework, which tends to be far less clear when a dispute actually arises.

Because a shared wall means your renovation plans can directly affect your neighbour's structural support, having a clear, registered party wall agreement, even if none currently exists, is a genuinely useful step before undertaking any work near that wall.