Every manufacturer of factory-built housing subject to requirements of this subchapter shall obtain plan approval for each model, and shall obtain insignia for each unit manufactured. The manufacturer shall maintain a quality control program, and the units shall be manufactured in accordance with the approved plans and shall be inspected as required by provisions of this subchapter. No factory-built housing bearing a department insignia of approval shall be in any way modified prior to installation unless approval is first obtained from the department or design approval agency.