First principle to remember, if your boss asks you to work overtime, you cannot refuse it. You will not be penalized if he warns you at the last moment and has not given you time to organize yourself. Otherwise the sanction is heavy: it is a reason for dismissal or even serious misconduct which results in the immediate termination of the employment contract without notice.
An additional hour corresponds to any hour worked beyond the 35 hours per week, a rule which obviously does not apply to employees on a day package and even less for senior executives. There are limits to overtime. First of all, working overtime shouldn’t cause you to exceed a limit. The maximum working time is 10 hours per day and 48 hours per week, or an average of 44 hours calculated over the last 12 weeks. Second limit: there is an annual quota of overtime in principle defined by the collective agreement which applies to your profession or by a company agreement. If such an agreement does not exist, we refer to the national value, which is 220 hours per employee per year. Unless this overtime gives entitlement to equivalent compensatory rest. In this case, we can exceed the annual quota.
There is a minimum. The law provides for an increase of 25% for the first eight hours and 50% for the following ones. Collective agreements may provide for another arrangement, but overtime must be paid at least 10% more than ordinary hours. But the remuneration of overtime may be replaced, in whole or in part, by equivalent compensatory rest. For example, if overtime is paid in a company 50% more, an employee can be granted 1h30 of rest per hour worked more. Please note that overtime worked in 2020 is not subject to tax up to the limit of 5,000 euros per year.
On Sunday work, the principle is voluntary service. We cannot force an employee to come to work on a Sunday and if he does, there is a salary compensation. It depends on the company agreement that has been signed, but in practice there is often a 50% increase in hourly wages.