Can provincial rules override a municipality's zoning bylaw to allow additional residential units?

Yes, in this specific area, Ontario's provincial planning framework has directed municipalities to permit additional residential units as-of-right in many residential zones, reflecting a broader provincial housing policy objective, and municipal zoning bylaws are generally required to be updated to reflect that direction rather than being free to prohibit these units outright. Where a municipal bylaw doesn't yet properly reflect the required provincial standard, the provincial direction is generally intended to take precedence over an outdated or non-conforming municipal restriction.

That said, this provincial direction generally sets a floor establishing that these units must be permitted, rather than eliminating all municipal control over the specific details, since municipalities generally retain the ability to set particular standards within that framework, such as size limits, parking requirements, and servicing conditions, so long as those standards don't effectively defeat the provincial objective. Because municipalities update their bylaws to reflect provincial direction on different timelines, and the specific scope of what's protected can be technical, anyone relying on this kind of provincial override for a specific project should get a clear, current opinion on how it applies to their particular municipality and property, rather than assuming a general rule automatically resolves every local detail.