You first issue a legal notice to the builder to remove the temporary toilet built by him for the facility of the workers by demolition and also mention that the toilet is not finding a place in the approved plan, moreover it is causing inconvenience hence he should immediately remove the same as was assured by him at the time of construction of the villa.
You can endorse a copy to the association with a request to instruct the builder to remove the same immediately or to demolish the same by taking a decision through a meeting of the association.
If all these steps do not fetch you any results in your favor, you may initiate legal steps by instituting a mandatory injunction suit to remove the illegally constructed toilet in the open place contradicting the rules and approved plan.