Any lot in the RA-3 and RA-5 zones, which had an existing structure prior to April 9, 2001 and is on an undersized lot, may have a zoning permit issued for a permitted use without an appeal to the land use board provided that the building yard requirements are met for the R-40 zone. On lots 20,000 square feet or less, the building yard requirements for the R-10 zone shall apply.