Any building permit issued prior to the effective date of this section for work that does not comply with this chapter shall continue in effect, but, in case no work under such permit should be commenced before it expires (i.e., before its original expiration date or the date to which it may have been extended by the Board of Trustees) or within three months after the date of the adoption of this section, whichever date is the earlier, such building permit shall expire by limitation; provided, however, that such expiration date may be extended by the Board of Trustees. If work under such a building permit has commenced prior to its expiration date as herein provided, such work may be carried out and completed, and a certificate of occupancy or certificate of compliance, as the case may be, shall be issued in the same manner as if amendments to this chapter had not been adopted. This section shall not affect any building permit issued for work that complies with this chapter.