The maximum floor area of the principal dwelling on a lot shall be 4,000 square feet, excluding an attached garage of up to 250 square feet, except that for lots less than 1/2, acre (21,780 square feet), the permitted floor area for a principal dwelling shall be computed by multiplying the actual square footage of the parcel by the floor area ratio of 0.184, but no dwelling shall be required to be less than 2,000 square feet with attached garage up to 250 square feet.