Front Yard. Each lot in the R-1 district shall have a front yard extending (except for access drives and walks) across the full width of the subject property of a depth of not less than twenty feet; provided, however, that the planning commission may require staggering of setbacks, with a variation of three feet from setback lines. In no case shall a setback of less than seventeen feet be allowed, except on cul-de-sacs, where the planning commission may approve a setback of fifteen feet as a part of a total development plan. Except for access driveways, walks, fences, and minor ornamental structures, there shall be no structures located in the required side yards abutting a street. No boat or trailer shall be kept in said required front or side yard; nor shall it be permitted to dismantle, repair or keep any disabled vehicles in this front or side yard or driveway; nor shall storage of any material be permitted.