Unjust enrichment is a doctrine for the compensation of one who has unjustly received a benefit from another in a manner that the law will correct. It is a principle of “equity”.
Thomson-Carswell’s Dictionary of Canadian Law (3d ed) neatly summarizes the doctrine:
“ An action for unjust enrichment arises when three elements are satisfied: (1) an enrichment; (2) a corresponding deprivation; and (3) the absence of a juristic reason for the enrichment.”
Each of these is required for the doctrine to be applied. Only when they are proven is the action established and the court able to grant compensation; from there the court goes on to determine what remedy will be applied: monetary compensation or a judgment finding a “constructive trust” which will result in an order that the claimant has an interest in or title to property.
An example of unjust enrichment would be two companies working together without a written contract on a building project where Company A owns the land and Company B contributes labour and building supplies. Co.A refuses to pay Co.B, so where is B’s remedy? It can allege an oral contract, yes. But it can also allege that Co.A is unjustly enriched. Co.A has been enriched to the value of the contributions in labour and supplies and through any increase in the value of the property. Co.B has suffered a corresponding deprivation, in that it has lost those contributions and the profit (the increase in value) which would have been provided if those contributions had been used elsewhere. Lastly, there is no juristic reason for what happened. If, for example, Co.A had been owed $1m by Co.B and B’s deprivation was roughly the same, Co.A could claim a set-off for that debt, which could be a juristic reason which may be upheld by the court.
For further reading and a discussion of the principle, please see the Duhaime page on unjust enrichment.
UPDATE, 2018-11-28: The Supreme Court of Canada has produced a new and important unjust enrichment case: Moore v. Sweet 2018 SCC 52 . John O’Sullivan has produced a good summary of that case here.