When do I get my retention money back?

Retention - the slice of each payment held back as security that faults get fixed - comes back in the way your contract says, and only there: commonly a first release when the build is handed over, and the final release months later when the defects period - the run of months in which the builder must return and fix faults - comes to an end. Two failures cost self builders here. The first is releasing early out of politeness, which buys nothing back if a fault appears the following month. The second is simple forgetting: the final date arrives, nobody walks the house or claims the money, and the sum quietly stays where it is. Put both dates in your diary the day the contract is signed, book a walk-round a month before the period ends, log anything that has appeared, and release when the contract says so - not before, and not never.