https://www.epo.org/en/node/what-happens-if-i-am

What happens if I am using the automatic debiting procedure and the EPO as ISA or IPEA considers that the invention claimed in my PCT application lacks unity?

In non-unity cases the EPO will invite you to pay additional international search or preliminary examination fees. Unless you inform the EPO otherwise, it will debit all such fees on the last day of the one-month time limit.

As a courtesy service, EPO will inform you separately that it has issued such an invitation.