The Wetland and Floodplain (WFP) District is considered to be an overlay district, superimposed on other districts. Any uses permitted in the portions of other districts so overlaid are permitted and may be continued, but no new buildings, structures, earth relocation or excavation or fill shall be permitted, except as is otherwise specifically permitted in the Schedule of Use Regulations and except that dwellings, farm buildings and buildings and structures accessory thereto may be expanded in ground coverage by not more than twenty-five percent (25%) compared to the area covered by all buildings on a lot as of October 8, 1985. The expansion of accessory structures shall require a special permit from the Board of Appeals, as shall the relocation or new construction of ways and the alteration of watercourses, water bodies and dams. In Wetland and Floodplain Districts, fill, earth relocation, new uses and structures or expansions thereof permissible in the underlying district may be permitted by the Board of Appeals by special permit upon referral to the Board of Health, Conservation Commission and Planning Board and a finding that the land is not subject to flooding or otherwise unsuitable for the proposed structures or uses and that, according to a certificate signed by a registered professional engineer, there will be no increase in one-hundred-year flood levels as a result of the special permit.