Rates shall be used, subject to the other provisions of this chapter, only if made in accordance with the following provisions:
(1) In the case of insurances under standard fire policies and that part of marine and transportation insurances not exempted under RCW
48.19.010, manual, minimum, class or classification rates, rating schedules or rating plans, shall be made and adopted; except as to specific rates on inland marine risks individually rated, which risks are not reasonably susceptible to manual or schedule rating, and which risks by general custom of the business are not written according to manual rates or rating plans.
(2) In the case of casualty and surety insurances:
(a) The systems of expense provisions included in the rates for use by any insurer or group of insurers may differ from those of other insurers or groups of insurers to reflect the requirements of the operating methods of any such insurer or group with respect to any kind of insurance, or with respect to any subdivision or combination thereof for which subdivision or combination separate expense provisions are applicable.
(b) Risks may be grouped by classifications for the establishment of rates and minimum premiums. Classification rates may be modified to produce rates for individual risks in accordance with rating plans which establish standards for measuring variations in hazards or expense provisions, or both. Such standards may measure any differences among risks that can be demonstrated to have a probable effect upon losses or expenses.
(3) Due consideration in making rates for all insurances shall be given to:
(a) Past and prospective loss experience within this state for experience periods acceptable to the commissioner. If the information is not available or is not statistically credible, an insurer may use loss experience in those states which are likely to produce loss experience similar to that in this state.
(b) Conflagration and catastrophe hazards, where present.
(c) A reasonable margin for underwriting profit and contingencies.
(d) Dividends, savings and unabsorbed premium deposits allowed or returned by insurers to their policyholders, members, or subscribers.
(e) Past and prospective operating expenses.
(f) Past and prospective investment income.
(g) All other relevant factors within and outside this state.
(4) In addition to other factors required by this section, rates filed by an insurer on its own behalf may also be related to the insurer's plan of operation and plan of risk classification.
(5) Except to the extent necessary to comply with RCW
48.19.020 uniformity among insurers in any matter within the scope of this section is neither required nor prohibited.