Within six months after the approval of the Board of Trustees for a permit for a swimming pool, the applicant shall obtain a building permit for the construction of said swimming pool, and the construction of said pool shall be completed within one year after the approval of the permit by the Board of Trustees. Upon written application made prior to the expiration of such periods of time, the Board of Trustees may grant an extension of time for such acts, provided that no such extension shall continue more than six months from the date originally provided for the completion of such act. In the event that the applicant does not obtain the building permit and complete the swimming pool within the required period of time, the approval of the Board of Trustees shall expire.