In Brazil, administrative urban planning law constitutes a privileged field for observing developments in the dialectic between the principles of legal certainty and legality. The equation has tended to be resolved in favour of the former. Given the need for stability expressed by various operators in the construction sector, Brazilian legislators and judges have adopted an arsenal of mechanisms to maintain an act or a situation in the legal order, whether to prevent the imposition of legal penalties or the sole prospect of appearing before the judge.