One additional key will be issued to an individual who completes an entry form for any Redrow development in South Wales.
Only one of the 20 keys will open the door to the flat Redrow is giving away. The details of who has it will be discovered on February 29 when all keyholders will test their keys at events covered by Real Radio's outside broadcast team.
The promotion offers Redrow exposure for its 'Debut by Redrow' first-time buyer range. Listeners are offered the chance to text 81056 for an extra clue each day, and the clue will come with more information about Debut attached.
Ricky Durkin, programme director at Real Radio Wales, claimed the competition was the biggest ever to run on Welsh radio.
"This is one of those rare occasions where we really can say, this is a life-changing competition," he said.
The competition comes as housebuilders contend with a deteriorating outlook in the property market. The number of new mortgage loans approved in December was the lowest since figures started being collected in 1997, according to the British Bankers' Association.
This article was first published on brandrepublic.com