House Democrats warned the Federal Communications Commission yesterday not to adopt a rate plan that would bill local users for most of the additional costs incurred by local telephone companies once they separate from the Bell System.

The extra costs, totaling an estimated $10.2 billion, have been borne by American Telephone & Telegraph Co., but will become the local companies' responsibility after their separation from AT&T in January 1984.

If these costs are passed to local telephone users, their bills would increase by a nationwide average of $7 a month and would do violence to the concept of affordable telephone service for everyone, Rep. Timothy E. Wirth (D-Colo.), chairman of the House telecommunications subcommittee, said at an FCC oversight hearing.

FCC Chairman Mark S. Fowler said that the FCC, through a joint board of three FCC members and four state communications regulators, has been examining four approaches to recovering local company costs.

Two plans would allow cost recovery through rates charged for long-distance service. The other two are called the Pure 2 because they would pass the costs of the separation directly to local telephone users.

The commission is scheduled to decide on one approach or a combination by Dec. 30.

"I am profoundly troubled by the Pure 2 access charge you apparently are about to adopt," Wirth told Fowler.

"Pure 2 goes too far, too fast, even if it is phased in over several years. It is a cure that is all out of proportion to the problem," Wirth added.

AT&T has argued that its local companies were largely supported by AT&T's long-distance service income. That particular claim has been the subject of much debate. But there is general agreement that, however it was done, much of the local companies' costs were absorbed by the parent.

Fowler insisted that the FCC has not reached a decision and that any solution will be aimed at "keeping as many people on the network as possible.

"The commission's major objectives . . . are to aid in the promotion of technically- and cost-efficient service and pricing without harm to universal service -- goals which are difficult to reconcile, at least in the short run," Fowler said.