Hammerhead lots may be created and excluded from the existing street limitations, provided that the Planning Board authorizes the creation of the lot by special permit. Hammerhead lots shall have at least forty-foot frontage, five acres' area, a two-hundred-foot building setback from the street line and twenty-five-foot setbacks from all side and rear lot lines and shall comply with § 218-6.3G, Computation of lot area. Reduction in frontage shall not be allowed where likely to result in a hazardous concentration of egress points. Hammerhead lots may be built upon for single-family and permitted nonresidential use.