Serious crime in the District of Columbia increased 6 percent in the third quarter this year over the same period last year with major increases in buglary and auto theft, D.C. police reported yesterday.

The increase brought to an end a 22-month period of declining crime reported in the city.

Although crime increased in all seven major offense categories, the pattern was uneven throughout the city. It ranged from a reported overall increase of 12 percent in the affluent and commercially expanding 2nd Police District west of Rock Creek Park to a decrease of 6 percent in the 7th District in far Southeast, an area of freeways and low and middle-income housing.

For the city as a whole, a total of 14.145 serious offenses - homicide, rape, robeery, aggravated assault, burglary, larceny and auto theft - were reported to the police during the July-September quarter this year. There were 13,329 in the third quarter of 1977.

At a press conference yesterday, Police Chief Burtell Jefferson blamed the increase on a number of factors, including unemployment, inflation and the poorer quality of street drugs, especially heroin, which forces addicts to steal more in order to purchase greater quantities of drugs to satisfy their habit.

Jefferson also said that an improvement in police-community relations has had the effect of causing more crime to be reported to police, pushing up the officials statistics.

"We have a better working relationship with the community," Jefferson said, "and people are reporting more crime to us . . . We have the confidence of the community."

Police statisticians acknowledge that many crimes are not reported by victims. A 1974 Justice Department victimization survey of the nation's five largest cities - New York, Chicago, Los Angeles, Philadelphia and Detroit - showed that the percentage of victims reporting crimes ranged widely from nearly 100 percent robbery victims to less than 25 percent of larceny victims. Victimization figures for Washington were not immediately available.

The reasons for not reporting crime also varied - inconvenience, fear of reprisals, personal concerns - but the major reason, according to the survey, was lack of confidence in the police and a belief that nothing would be done.

Of the seven major crime categories, reported auto thefts showed the greatest increase - 31 percent - in the third quarter this year in Washington.

Jefferson attributed the reported increase from 702 to 920 stolen cars in part to "increased sophistication" of car thieves who have learned how to circumvent the steering column locking mechanisms on late-model cars.

Reported burglaries, which shot up 12 percent from 3,259 to 3,644, stemmed largely from break-ins at new office building developments throughout the city, but especially in the 2nd District, Jefferson said.

Homicides increased 18 percent from 51 to 60, reported rapes 9 percent from 100 to 109, reported robberies 4 percent from 1,544 to 1,602 and reported larcenies 2 percent from 6,960 to 7,077, according to police figures.

The 1st Police District, embracing downtown and parts of Capitol Hill, reported an overall 3 percent crime increase. The inner city 3rd District reported a 10 percent increase, while the 4th District, including upper 16th Street and Georgia Avenue NW, reported a 7 percent increase.

The 5th District, which includes the New York and Rhode Island avenue corridors in upper Northeast, and the 6th District east of the Anacostia River in far Northeast, both reported an 8 percent increase overall.