With accordance to article 6 of the General Data Protection Regulation (GDPR), there are six available lawful bases for processing data and you must have at least one valid lawful basis in order to process personal data from your clients.
(a) Consent: the individual has given clear consent for you to process their personal data for a specific purpose.
(b) Contract: the processing is necessary for a contract you have with the individual, or because they have asked you to take specific steps before entering into a contract.
(c) Legal obligation: the processing is necessary for you to comply with the law (not including contractual obligations).
(d) Vital interests: the processing is necessary to protect someone’s life.
(e) Public task: the processing is necessary for you to perform a task in the public interest or for your official functions, and the task or function has a clear basis in law.
(f) Legitimate interests: the processing is necessary for your legitimate interests or the legitimate interests of a third party, unless there is a good reason to protect the individual’s personal data which overrides those legitimate interests. (This cannot apply if you are a public authority processing data to perform your official tasks.)
It is important to note that it is better to avoid collecting information you don’t need. In addition, don’t store information any longer than you need to. Your data protection expert or legal adviser can help you on what legal basis to rely on and what information to collect.