There is no obligation for applicants to pay any additional fee.
However, subject-matter that has not been searched will not be examined by the examining division (G 2/92). Hence, it cannot be prosecuted in an independent claim.
If the lack of unity persists also in examination after the claims have been amended, the excision of the non-searched subject-matter from the application may be necessary (see C‑III, 3.2, and F-IV, 4.3).
Non-searched subject-matter can always be prosecuted in a divisional application.
Source: http://www.epo.org/law-practice/legal-texts/html/guidelines/e/f_v_4_2.htm
Date retrieved: 17 May 2021