In French law, the delay is governed by contract law, by the contract and by case law. Each kind of delay is governed by its own rules.
Exemptions governed by contract law
The first case of exemption for the contractor in case of delay is an act of God (article 1218 of the French Civil Code). It is an unforeseeable, irresistible and external event. If the event is temporary, the execution of the contract is suspended and the contractor gets an extension of time (except if the delay is so important that the contract must be terminate). If the event is definitive, the contract is terminated without other formality.
Exemptions governed by the contract
Legitimate causes of suspension of time can be decided by the parties. They create their own acts of God in the contract. These legitimate causes of suspension lead to an extension of time for the contractor. For example, bad weather, public holiday, strike, war, etc.
Sanctions governed by the contract and by contract law
If the delay is not caused by any aforementioned cause of exemption, it is a contractual fault by the contractor governed by the French Civil Code and the contract.
The obligation to finish the works on time is essential in that an incorrect behaviour of the contractor has not to be proved.
The contractor is liable for damages in case of a contractual fault that causes a delay (article 1231-1 of the French Civil Code). The owner has to prove a fault, an injury and a causal link between the fault and the injury.
A penalty clause can be stipulated in the contract in case of delay. In such a case, an injury doesn’t have to be proved because the penalty clause stipulates a sanction for delay