If any party served with a notice of appeal wishes to participate in the proceedings on appeal, he must file an answer within 30 days after service on him of the notice of appeal or statement of reasons where such statement was not included in the notice of appeal. If additional reasons, written arguments, or briefs are filed by the appellant, the adverse party shall have 30 days after service thereof on him within which to answer them. The answer must state the reasons why the answerer thinks the appeal should not be sustained. Answers must be filed with the Board (address: Board of Land Appeals, Office of Hearings and Appeals, 4015 Wilson Boulevard, Arlington, VA 22203) and must be served on the appellant, in the manner prescribed in section 4.401(c), not later than 15 days thereafter. Proof of such service as required by section 4.401(c), must be filed with the Board (see address above) within 15 days after service. Failure to answer will not result in a default. If an answer is not filed and served within the time required, it may be disregarded in deciding the appeal, unless the delay in filing is waived as provided in section 4.401(a).