R. 137(2) EPC specifies that "Together with any comments, corrections or amendments made in response to communications by the European Patent Office under R. 70a (1) or (2) EPC, or R. 161(1) EPC, the applicant may amend the description, claims and drawings of his own volition".
Thus following receipt of the search report and search opinion, the applicant must (subject to certain exceptions) respond to the search opinion, by filing amendments to the description, claims or drawings and/or filing his observations on the objections raised in the search opinion. Any amendments filed at this stage are made by the applicant of his own volition in accordance with R. 137(2) EPC (Guidelines C-II, 3, version 2012).
A reply to the search opinion and any amendments of the applicant's own volition should be on file when the substantive examination phase starts (see R. 70a, 137 and 161 EPC) (CA/81/10 Rev. 1, R. 71). The applicant's response required by R. 70a EPC (or filed voluntarily in response to search opinions not requiring a response) will be taken into account by the examining division when drafting the first communication.
In T 1178/08 the board stated that in accordance with Art. 123(1) EPC in conjunction with R. 137(2) and (3) EPC, an applicant is to be given at least one opportunity to amend the application of his own volition. No further amendments may be made without the consent of the examining division. It noted that the EPC and the Implementing Regulations do not contain explicit provisions concerning the admissibility of amendments in appeal proceedings. R. 66(1) EPC 1973 sets out that the provisions relating to proceedings before the department which has made the decision from which the appeal is brought are applicable mutatis mutandis in such a case. Thus the board held that an appellant is not entitled, as a matter of right, to present amendments of his own volition at the appeal stage, but as a matter of discretion to be exercised by the board. Art. 12 and 13 RPBA set out the provisions governing the basis of appeal proceedings and amendments to a party's case which are binding upon the boards of appeal (Art. 23 RPBA). In T 979/02 the board found that by virtue of Art. 111(1) EPC and R. 66(1) EPC, these provisions (i.e. R. 86(3) EPC 1973) are also applicable to appeal proceedings mutatis mutandis.