Mann v Paterson is about what happens when a construction relationship breaks and someone says: pay me for the value of the work, not just the contract price. That sounds intuitive, but the High Court pulled the analysis back toward the bargain the parties actually made.
For builders and principals, the practical lesson is to make the contract do its job while the project is alive. Stage payments, variation rules, notices and termination steps matter because they can shape what is recoverable after the relationship collapses.