Much communicative planning is consensus oriented and rests on ideas of deliberative democracy. Planning recommendations made by dialogue are based on the intellectual force of arguments giving reasoned rankings of the planning alternatives. Dialogue encompasses the amalgamation of arguments in accordance with democratic criteria ensuring the communicative rationality of the process and the legitimacy of the recommendation. The balancing and weighing of arguments should avoid decision cycles that would make the recommendation of a plan arbitrary. By an analogy with Arrow’s theorem on the general impossibility of consistent and fair social choice, it is demonstrated that dialogue cannot ensure consistent recommendations and simultaneously prepare for political decision making in a democratic manner. The result is valid for debates over planning alternatives when differences in quality are not comparable across all the important arguments (concerning noise, safety, visual standard, social impact, etc.), which is the most common situation.
