An ALTERATION to a plumbing system where the total cost of the proposed work in the building does not exceed twenty five thousand dollars in any 12 month period and the proposed work is limited to the following:

1. The installation of new plumbing or gas piping, or the rerouting of existing plumbing or gas piping;

2. The addition of not more than two plumbing fixtures or fixture connections;

3. The mounting of new plumbing fixtures on existing roughings, other than the mere replacement of existing fixtures constituting a minor alteration or ordinary repair under this code; and

4. The installation or replacement of BACKFLOW preventers.