Ownership and income tax: implied trust: constructive trust - detriment
If (and only if) evidence of a ‘common intention’ is found, the claimant then has to show that he/she also acted to his/her ‘detriment’, or substantially changed his/her position on the basis of that common intention. Lloyds Bank v Rosset,  1 A.C 107 at 132 E-F:
‘Once a finding [of a common intention] is made it will only be necessary for the partner asserting a claim to a beneficial interest against the partner holding the legal estate to show that he or she has acted to his or her detriment or significantly altered his or her position in reliance on the agreement in order to give rise to a constructive trust or a proprietary estoppel.’
Detriment is an extra step that has to be proved, over and above an oral trust: Lloyds Bank v Rosset,  1 A.C 107:
‘Even if there had been the clearest oral agreement between Mr. and Mrs. Rosset that Mr. Rosset was to hold the property in trust for them both as tenants in common, this would, of course, have been ineffective since a valid declaration of trust by way of gift of a beneficial interest in land is required by section 53(1) of the Law of Property Act 1925 to be in writing. But if Mrs. Rosset had, as pleaded, altered her position in reliance on the agreement this could have given rise to an enforceable interest in her favour by way either of a constructive trust or of a proprietary estoppel.’
Detriment means that the claimant must have done something which he or she could not reasonably be expected to have done unless they were to have an interest in the property. For example, they may have given up a job and moved to another part of the country to assist a disabled relative. The detriment has to be significant.