if the property is not developed contrary to the exposé specifications, google Prospekthaftung Makler
First: the property is, in all likelihood (thread history, statement from the OP), fully developed, therefore there is no deception etc. by the real estate agent, but solely a (common) misunderstanding by the prospective buyers.
Second: real estate agents are _not_ subject to the liability under the prospectus for example of securities issuers. The term "Prospekthaftung" is in this discussion at least as misleading as the technical term "fully developed," because it usually refers to the liability of securities issuers etc. Correct is: agents are liable under certain circumstances (!) or in cases of gross violations for statements given to them by the sellers; however, fundamentally the agents only pass on the information of the owners.
In this case, the agent supplemented the offer with the addition "fully developed," thus made own statements. But if the property nevertheless has no connection to a public traffic area (street) and/or water, sewage, electricity, telephone, internet etc. – then the agent would have to be either quite an idiot or unbelievably brazen.
Of course, this is possible – but rather unlikely.
Best regards Dirk Grafe