Axwhale,
It depends, ‘denuncia’ just means report or formal complaint, a statement of the facts of a case…
It can be made before a watch or police court (I think this is a magistrate’s court in the UK), a public prosecutor or a police officer if you belive the facts to involve some kind of criminal activities.
You may also report a bulider/promoter before the administrative/consumer protection authorities if you think they have failed to fulfill their administrative obligations or they invade a legit right, or fail to perform a legal duty in a non-criminal liable way.
If they are in breach of contract you can also report them to the consumer protection authorities besides promoting a private civil action before a regular court
Cesar