Construction difficulties have forced Metro to delay for several months the planned July opening of the metro subway extension through northeast Washington to New Carrollton, general manager Theodore C. Lutz said yesterday.

Lutz, complaining that "we get hung by our own desire to be so specific" by predicting months in advance exactly when specific legs of the subway would open, cited four reasons for the latest delay:

Cold weather last winter delayed the pouring of concrete for several important structures on the line.

Extended negotiations were required with the railroad (first Penn Central, now ConRail and Amtrak) over access for construction crews. Much of the New Carrollton line parallels existing railroad tracks.

Negotiations also were required with the Potomac Electric Power Company over the relocation to some lines along the right of way.

Some extra time was needed to repair "discrepancies" found in the concrete work the Cheverly station.

Metro board chairman Francis W. White, who also is chairman of the Prince George's County Council, said the delay in opening the first segment of the subway to penetrate Prince George's would not cause the county undue difficulty.

Metro's construction schedule has been plagued with delays from the start in 1969. The most recent delay was in the opening of the Red Line extension from Rhode Island Avenue to Silver Spring. That was planned to take place last November, but is now scheduled for Feb. 3.

Lutz assured the board yesterday that the Silver Spring opening was still on the new schedule. Test trains have been running on that new segment for several months now.

Lutz also that a delay in opening the New Carrollton line would not impede Metro's ability to provide weeknight and Saturday subway service beginning and September. The Metro board budget committee has recommended such expanded service, but final action has not been taken.

The continual slippage in Metro completion dates has hurt Metro's credibility both with the federal government, which provides much of the construction money, and local citizens, who want the train to reach them.

Lutz suggested that Metro might want to adopt a policy of predicting completion dates within a fairly broad range, instead of a certain month. The present Metro schedule predicts to the month the opening date of each segment.

Lutz, in effect, adopted the new policy himself yesterday when he said the New Carrollton line would open between late October and late November, but most probably in mid November.

The New Carrollton line will be either elevated or at grade from the RFK Stadium parking lot along Benning Road. Two District of Columbia stations are planned at Minnesota Avenue and in Deanwood as the line bends to the Northeast.

Prince George's County stations are under construction in Cheverly, Landover and Newr Carrollton.The New Carrollton station will be combined with a new Amtrak station so that passengers will be able to make quick connections between Amtrak and the Metro trains.