Such fees shall be due and payable upon application to the City for a building permit and plan check for the construction of any such dwelling unit or the construction of a mobile home park, or addition thereto; provided, however, that there shall be a refund of such fees in the event the building permit is not approved, or is not used, for such construction. Fees are due before any construction or excavation permit is issued for any new or addition to existing mobile home park.