This study adopts a discrete choice modelling methodology to evaluate individuals’ preferences over planning alternatives for an urban site of environmental interest. Since such projects involve some uncertainty and irreversibility, a special attention is devoted to the estimation of the quasi-option values which are associated to project development. Two distinct measures for the quasi-option value are estimated, and both coefficients indicate that the public places a significant value on reduction of the possibility of adverse irreversible effects: a more prudent development strategy is valued about four times more than a procedure that provides a lesser hedge against undesired outcomes. Furthermore, the study involved elicitation of intertemporal preferences over projects with different time spans, and estimation of the implicit discount rates: the values obtained seem high if compared to standard discount rates applied to public projects, but not far from current interest rates on consumption.