Does an Ontario seller have to disclose a known mould problem before selling?

Mould that is visible and detectable on a walkthrough is generally treated as a patent defect, meaning the general rule of buyer beware largely applies and a buyer is expected to have caught it through a reasonable inspection. Mould hidden behind walls, under flooring, or in a basement corner a buyer wouldn't normally examine is a different matter: if the seller actually knows about it and knows it makes the home unsafe or unfit to live in, Ontario law generally requires disclosure of that specific, known, hidden condition.

The tricky part is that mould often has both a visible symptom and a hidden underlying cause, such as a leak or ventilation problem, and a seller may genuinely not appreciate the full extent of what's behind a small visible patch. Buyers should treat any sign of past water damage, musty odour, or staining as a reason to ask direct written questions and consider a specific moisture or mould inspection rather than relying on a general home inspection alone, since remediation costs can escalate significantly once mould has spread inside wall cavities or subflooring.