RDJ in a French real estate advertisement is an abbreviation for Rez de Jardin.
Rez de Jardin is the part of a building that is on the same level as the garden. For example, if you have an French apartment that is on the ground floor and when you walk out from the living room of the apartment you are in the garden this apartment is typically advertised as RDJ or Rez de Jardin.
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