What are my rights over a shared wall in a semi-detached home if there is no formal party wall agreement in Ontario?

Without a formal party wall agreement, your rights and obligations toward a shared wall come from general common-law principles rather than a specific written framework, since Ontario has no general statute setting out default party wall rules the way some other jurisdictions do. In broad terms, both owners are treated as having reciprocal rights of support and use in the shared wall, along with a mutual obligation not to damage or endanger the other side's portion or the wall's structural integrity.

What this means practically is that neither owner can unilaterally demolish, remove, or materially alter their side of the wall in a way that takes away necessary structural support from the other unit, without the other owner's consent. Doing so without consent, and causing damage, can expose the responsible owner to liability even without a written agreement in place. But because these principles aren't spelled out in a specific document, exactly how repair costs get shared, or what counts as an unacceptable alteration, can become genuinely unclear once a real disagreement arises.

Given how much ambiguity exists without a written agreement, putting a registered party wall agreement in place, spelling out maintenance, cost-sharing, and notice requirements, is a sensible step even for an existing semi-detached home.