The net cost of buying a house may increase throughout the country by an average of 13.3 percent this year, according to a new study by Chicago Title Insurance Co.

Inflation in house prices will account for about 6 percent of the rise and increased borrowing costs because of higher mortgage rates will account for 7.3 percent, the property title insurer said.

In the Washington area, the company said that overall costs are likely to increase 12 percent.

The expected increase this year contrasts with a net decrease of 6.3 percent last year, a period of generally declining mortgage rates. The company said that since the housing recovery period began in 1982 there has been some home price inflation every year, but that net housing costs since then have declined by 3.3 percent as mortgage rates have generally dropped.

"This will be something of a catch-up year for housing-cost gains," said John Pfister, vice president and manager of market research for Chicago Title.

In a related survey, the Mortgage Bankers Association of America reported that a mid-July telephone poll of 20 large mortgage companies showed that as mortgage rates rise, as they did last spring, about 11 percent of prospective home buyers decide to postpone their purchase. Other borrowers look for alternative financing, such as starting with a lower-rate adjustable mortgage and then converting it later to a fixed-rate mortgage, the trade group said.

Meanwhile, the Commerce Department reported that expenditures for upkeep and improvement of residential properties in the first quarter of 1987 amounted to a seasonally adjusted annual rate of $89.7 billion, slightly higher than the fourth quarter 1986 rate of $88.9 billion.

Actual first-quarter expenditures totaled $17.7 billion, $10.1 billion of it spent by owner-occupants of residential properties. Estimated net operating income for office buildings in North America dropped by 2.2 percent last year, the first such decline during the economic boom of the past few years, according to the Building Owners and Managers Association International.

"Sluggish local economies coupled with the specter of tax law changes and dramatically higher leasing expenses hit the bottom line of the U.S. office building industry in 1986," said Noel Leary, executive vice president of BOMA International. "Last year's losses in net operating income are causing the entire industry to place a higher premium on building management and financial planning."

The trade group said that retail occupancy levels remained stable, but that office space occupancy continued to erode in the face of the glut of office space in many cities in the United States. The fight for new office tenants is reflected in the 103.3 percent rise in leasing expenses over the past five years, BOMA said. Fixed expenses also helped to erode net operating income, the trade group said. Insurance and real estate taxes rose by an average of 4.8 percent, which for a typical 200,000-square-foot building meant a $30,000 increase in annual expenses.

The BOMA conclusions were based on a survey of 4,000 office buildings with 700 million square feet of space in 133 North American cities.

IN THE BUSINESS ... Long Homes Inc. of Woodbridge and Signature Communities Inc. of Chantilly, both of which build homes mostly in Northern Virginia, have decided to merge into Long Signature Homes Inc. effective Oct. 1. ... PRC Realty Systems of McLean has bought Real Time Services and Real Time Systems, both of Orange, Calif., which supply computerized information systems and publishing products for the real estate industry. ... Sherman A. Vining, W. Thomas Lock Jr. and Joseph Conrad have formed a new commercial property management firm called Aremco. ... Lerner Enterprises and Senate Construction have started construction on One Vintage Park, the first of five office buildings they plan at the 32-acre Loudoun Gateway Center at Dulles. ... Phillips/Stevens & Associates Inc. has started construction on the last of three buildings at the Burtonsville Office Park complex. ... Christopher Cos. have started construction on what it calls a charity house at Sully Station in Centreville. Christopher is donating proceeds from the sale of the first production house at the development to two youth charities, Residential Youth Services and Rainbow Christian Services. Christopher has pledged a minimum of $25,000. ... Cardinal Industries Inc., a large modular home builder, has filed two suits charging two competitors with copyright infringement and unfair competition. One suit was filed in federal court in Atlanta against U.S. Home Corp. and the other in Nashville against a joint venture of Basic American Realty Associates of Indiana and Rutherford Development Co. of Murfreesboro, Tenn. Cardinal is seeking damages and an injunction to block use of its floor plans and architectural designs. ... Mount Vernon Realty has opened a West Virginia office in Charles Town, 60 miles west of the District. ... Construction contracting jumped by 9 percent in June after two months of decline, according to the F.W. Dodge division of the McGraw-Hill Information Systems Co. June's annualized construction contract value was $253.6 billion. . . Seven metropolitan areas built more than 100 million square feet of office space in the 1981-86 period: Los Angeles (372 million), Dallas (206 million), San Francisco (191 million), Houston (156 million), New York (127 million), Chicago (110 million) and Washington (105 million), according to Commercial Construction Report.

PERSONNEL FILE ... Sargis & Jones, a commercial interior project management and general contracting firm, has promoted Ralph A. Dietze to vice president. ... George F. Allen has been named executive vice president of Capital Homes Inc. He has been the firm's chief financial officer since 1984. ... Michael M. Bequette has been promoted to vice president of property management at Epic Realty Services Inc. in Falls Church. ... GA/Partners Inc., a real estate counseling firm, has named R. Bruce Gamble as vice president.