The average family in Cambridge will pay $56 a year more for its water and sewage facilities starting next month, because of a plan approved by the because of a plan approved by the City Council at its meeting Monday night.

According to a report that City Manager Robert W. Healy presented at the meeting, the City's water utility expenditures will increase by an estimated 43 percent in fiscal year 1992 and its sewer utility expenditures will likely increase by 38 percent. As a result, the city must up the water bill to $291 for the average family, Healy said.

City Treasurer James Maloney attributed the increases to higher costs levied by the Massachusetts Water Resources Authority (MWRA). "The MWRA rates are essentially out of control," he told the council.

But Maloney said that the new plan, which measures fees in units of 100 cubic feet of water and effectively grants discounts to minor consumers, "shifts the onus to large user ships" like city universities and factory complexes.

Water and sewer fee price increases generally force citizens to conserve, Maloney added. He cited figures from fiscal year 1988, when a large price jump resulted in a 17 percent drop in consumption.

But Councillor Francis H. Duehay '55 said that he resented that Cambridge citizens are forced to pay for increased water usage in Boston, most of which, he claimed, was going towards the Boston Harbor clean-up. "We have a disappearing act going on of no further money to clean up our sewers," he said.

Duehay also said that the price increases will fall especially hard on the city's senior citizen population, because the increase in the discounts that qualifying elders have traditionally received will not keep pace with the overall rate increase. "We're cleaning up the rivers and streams in this country on the backs of the senior citizens," he said.

However, Duehay admitted that because of the state's bleak financial situation, Cambridge cannot avoid the price rises.

In other business, Healy reported to the council that he has created an interdepartmental working group to advise him on the Housing and Rent Control proposals currently under consideration by council subcommittees.

Members of the city's Housing and Rent Control subcommittees unveiled the proposals at the end of last month to offer some solutions to the city's perennial rent control and affordable housing debate. At the time, Councillor Edward N. Cyr called the housing proposal "arguably the most significant reorganization of city housing policy since the creation of the rent control system."

The proposals, which may be amended by the subcommittees, will go before the full council within two months, Councillor Jonathan S. Myers said Monday night.

The advisory group, which will deliberate with Healy on the feasibility of the measures set forth in the two proposals, will consist of the deputy and assistant city managers as well as city department heads involved in housing activities.