I'm not sure of the reason for your question, it sounds theoretical? I'm not a lawyer but I have been involved with assisting implementing the GDPR. If you were a window cleaner I would assume you have a contract to clean the windows (which might be oral or implicit rather than a formal written one). I'd take the stance that under Article 6 of the GDPR (Lawfulness of Processing), subsection 1.b "processing [of customers' data] is necessary for the performance of a contract to which the data subject is party or in order to take steps at the request of the data subject prior to entering into a contract". (You've been asked to clean their windows?) I'd argue that to fulfill any window cleaning services you need to know where your customers live. I'd guess you might also need to keep a record of customers if HMRC wanted to know the source of your income, i.e. who paid you? However, you do have an obligation to keep the customers' data private and accurate. It could be argued that obfuscating the street name/s is a basic way of protecting the data? You would probably need explicit consent if you were going to communicate anything to the customers another than anything other than fulfilling your window cleaning services. As I say, I'm not lawyer, so perhaps one will respond and either confirm my understanding or negate it!
Michael Ixer ● 556d