The average new four-bedroom, detached house in the Washington area went on sale last January for $88,700, an increase of about 16 percent over the January 1977 average, according to a report released yesterday by the Metropolitian Washington Council of Governments.

The other major type of new housing sold in the area - a three-bedroom conventional townhouse - increased in average price during that same 12-month period about 11 percent, from $55,390 to $61,282, according to Fred Selden, housing planner for COG.

The COG report used information provided by Housing Data Reports Inc., a local housing market analysis and consulting firm. It was based on a survey that included 316 major subdivisons in the area, representing 1,572 new housing models advertised for sale last January, Selden said. Generally, advertised sales prices in the area are "firm," Selden said - there's usually "no kind of reduction (in the sales price)" unless a particular type of model being offered is not selling at all, he added.

COG found that the average sale price for all types of new homes in the Washington area last January was $66,080.

Average sales prices varied greatly among jurisdictions, the survey found. For example, comparing prices for a new three-bedroom, conventional town house, average prices ranged from $142,000 in Arlington County to a low of $46,245 in Loudoun County. Selden noted that Arlington's average prices often rank high because there are few new subdivisions there, and those that are built sometimes are higher-priced, luxury buildings.

Not only are the prices of new houses increasing steadily, but home purchasers recently have had to confront higher mortgage rates. The Federal Home Loan Bank Board annouced this week that the national average mortgage interest rate for new homes rose last month to 9.46 percent, the highest rate since the government began compiling statistics in 1963.

Locally, home buyers have had to deal with interest rates at or near 10 percent, and some savings and loan associations in the area have reported recently that they either have stopped accepting new mortgage applications or are limiting the kind of applications accepted because of the tight money market.

For example, Perpetual Federal Savings and Loan Association has restricted its leading to customers who have savings accounts there, according to Thomas Owen, president of Perpetual.

Owen said that within the past two to three weeks, he has noticed a "marked decrease" in the demand for loans as interest rates have climbed.

"Ten percent is a rate that the public finds unacceptable," Owen said. "People will put off borrowing until the rates go down. People back off committing themselves to long-term indebtedness at these rates."

Other area savings and loan associations report that they, too, have begun to limit the number of mortgage they are providing.

For example, Independence Federal Savings and Loan Association has not been accepting applications for new mortage loans for about a month, said Hubert Merrick, executive vice president.

Columbia Federal Savings and Loan is accepting applications for owner-occupied properties but is not accepting applications now for investor loans, said Walter Hall, vice president. At Capital City Federal, loan amounts have been limited, said T.P. Nottingham Jr., vice president, and the savings and loan has stopped accepting applications for some types of loans.

In its survey of housing prices, COG found that in 1977, the trend was toward the production of smaller, more moderately priced single-family detached homes. But data for 1978 showed that the trend did not continue. The construction of large five-bedroom houses increased, it found, while fewer smaller homes were available.

COG's survey included single-family detached homes, conventional townhouses, "nonconventional" townhouses such as duplexes, low-rise and high-rise condominiums, and low-rise and high-rise condominium conversions. Houses ranged from condominium efficiencies to five-bedroom homes.

The survey found that the avarage sales price of a two-bedroom high-rise condominium last January was $62,608 in Fairfax County, $89,777 in Arlington County, $55,431 in Alexandria, and $110,000 in the District of Columbia.