Telephone subscribers who dial 411 for information in the District of Columbia more than six times a month will start paying extra for the excess calls within the next six to eight weeks.

The D.C. Public Service Commission granted final approval yesterday to a request by the Chesapeake & Potomac Telephone Co. To levy the charges. A C&P spokesman said he company will soon file necessary documents to put them into effect between Sept. 15 and Oct. 1.

On the date the new charge goes into effect, C&P will reduce the monthly phone rates by 34 cents for each of the city's 280,000 residential and 360,000 business and government subscribers.

Web Chamberlin, the company's spokesman, said that represents the entire estimated savings that will result from a reduction in the volume of information calls.

Those who make more than six information calls by dialing 411, the regular information number, will be charged 10 cents for each of those additional calls. Those who dial the central operator and must be switched will be charged 20 cents.

Calls from telephone booths, where printed directories often are not available, will be exempted. So will longdistance information calls.There will be no charge for handicapped patrons chiefly the blind and those who cannot use a telephone directory -- for whom special arrangements will be announced later by C&P.

A business firm that has several trunk lines feeding into a private branch exchange switchboard (PBX) will be permitted six information calls for each trunk line.

A business firm that has a centrex direct-dialing system will be permitted only three-quarters of one information call each month for each of its centrex numbers. In other words, the first call will cost about 3 cents, and all others will cost 10 cents.

C&P said such charges for information calls are permitted currently in 30 states, including Virginia. Under he terms of a new law in Maryland C&P has applied for permission to levy charges for all information calls in excess of 12 per month.