(a) Extraordinary cost means the cost, as determined by a qualified professional and shown in a written estimate signed by that professional, which also shows all relevant license numbers or professional qualifications, of installing equipment necessary to eliminate a shared meter in a dwelling or part of a dwelling which is in excess of the amount of rent for four months rental of such dwelling, or two months rental of such dwelling if the amount of service measured by the shared meter, that is used outside the shared meter customer's dwelling, is less than 20 percent of the average total monthly consumption recorded on the meter for the immediately preceding 12-month period (or if insufficient history is available, based on the best available information).

(b) Qualified professional means a person who is permitted to install or repair gas, steam or electric equipment, including rewiring or repiping, by the local code of the municipality where the meter in question is located.

(c) Legal impediment means a restriction which prevents separate metering, rewiring, or repiping due to zoning ordinances, which limit the number, type or location of meters in a building, or due to the historical significance of the structure, or any other legal prohibition against elimination of the shared meter condition.

Whenever the quantity of service on a shared meter, that is used outside of the tenant's dwelling, is estimated to be less than 10 percent of the total monthly consumption recorded on the meter, based on average monthly service for the immediately preceding 12-month period (or if insufficient history is available, based on the best available information), or 75 kwh/month of electricity, five therms/month of gas or one mlb/month of steam, whichever is greater, the owner, tenant and any third party whose utility service was to be measured through another meter and had caused or benefited from the shared meter condition, may negotiate a mutually acceptable agreement for the responsibility for the cost of such shared meter consumption. When the interested parties are unable to negotiate a mutually acceptable arrangement, the commission or its designee may order a remedy for a shared meter customer that it deems fair and equitable based on costs incurred and benefits received by the various parties.