Record-high mortgage rates, which flattened residential sales during the closing months of 1980, are expected to moderate somewhat early in 1981 -- thus stimulating home purchases. But the recovery in both new and existing home sales is expected to be slow until late spring.

Meanwhile, area-wide development of new office and light industrial buildings is expected to continue fairly strong in response to exceptionally heavy leasing demand in downtown and some prime suburban areas. However, the recent ultra-high level of prime borrowing rates has put some new development plans "on hold." This is due primarily to the heavy costs of carrying current projects on which construction loans are tied to the prime rate.

Until long-term home mortgage rates take a downturn below the 14 percent level, perhaps sometime this year, residential sales are likely to be soft in this area. The influx of several thousands of new Reagan administration personnel on both high- and medium-income levels is regarded as one of the few positive signs for both new and resale home marketing at this point.

Area realty professionals agree that a strong demand for housing ownership has been dammed up by excessive mortgage rates in recent months. "But there could be another strong rush into the market for both new and existing homes if mortgage rates fall below 13 percent," said Kenneth Murphy, a marketing executive with Long & Foster Realtors.

Likelihood of continued inflation and a shortage of new homes prompted builder-developer Michael T. Rose to predict that prices of new houses will increase fourfold by 1991. Rose, who builds moderately priced houses in Prince George's County, contends that a shortage of new homes in Montgomery, Fairfax and Prince George's counties and continued high borrowing rates will cause today's average $110,000 new house to cost $440,000 by late 1990.

Not unlike other builders who predicted earlier that new-home prices in the 1960s and 1970s would seem unrealistically low 10 years later. Rose insists that today's new home prices have nowhere to go but up. He pointed out that the cost of roofing, shingles, asphalt roads, steel window rims and appliances rise with oil prices because petrochemical products account for more than 50 percent of housing materials.

Meanwhile, many home resales are being made with the use of "wraparound" financing that involves creation of a new loan for the buyer and the continuation of a VA or FHA loan (both of which are routinely assumable) at a rate below market. William Ellis, head of residential sales for Shannon & Luchs, which set a record for total sales in 1980, said this combination of new and existing financing gives the seller cash and the buyer an overall rate below the current market.

While some long-term, fixed-rate mortgages are still being made at high rates, with the payments of discount points to increase yield and provide immediate cash return, there remains uncertainty that these conventional loans will continue to be readily available in 1981 and thereafter. High rates paid for six-month certificates in denominations of $10,000 and $100,000 have forced thrift institutions to seek higher returns on new mortgages.

One method of avoiding portfolios of below-market mortgages in the future is to use the renegotiable or variable rate mortgage, wherein lenders can adjust rates upward, periodically, with the market. To date, however, public acceptance of those changing rate mortgages has been minimal. One inducement is a lower rate base in the beginning for the borrower.

Prospects for new residential construction in this metro area are regarded as better in 1981 than the totals produced in 1980, when the housing market suffered two down cycles within nine months.

Statistics gathered by the National Association of Home Builders indicate that area single family starts will rise from the estimated 12,600 total in 1980 to about 15,000 in 1981. Multi-family starts are seen increasing from 3,670 in 1980 to 4,500 in 1981. Thus, the total starts estimated for 1981 would be 19,500 as contrasted to only 16,270 in 1980.

In must be recognized that the estimated total of housing starts here in 1981 would be higher than the 1980 figure but still slightly below the total in 1979 and substantially below the totals in 1977 and 1978.

Nationally, NAHB economists are less bullish than other housing observers. Some housing analysts earlier predicted total U.S. housing starts in the range of 1.5 to 1.7 million in 1981. However, NAHB staffers are intimately aware of the deadening effects of high borrowing rates on member builders across the land and thus are now predicting total starts below 1.4 million in 1981. That would be only a slight improvement over the low level of 1,280,000 housing starts in 1980.

Dispite record-high borrowing rates for construction loans, preliminary site work is under way on a large cleared site at Connecticut Avenue and L Street NW, where developers T. N. Lerner and Albert Abramson are moving ahead on a $100 million retail-office project named Washington Square.

Designed by architect Chloethiel Smith, this complex will have a million square feet of space in a spectacular glass atrium setting that is expected to set new highs in prices for both retail store and office space. Store rents are expected to average above $50 a square foot and office space to exceed $30 a square foot. Completion is scheduled late in 1982.

The Oliver T. Carr Co., which has been the No. 1 vital force in new office space construction in downtown for nearly a decade, plans to start half a dozen new buildings this year if construction loan rates moderate. The Carr firm plans to build its giant International Square complex in midtown and also do other buildings, including one at Canal and Ivy streets in the Southeast section of Capitol Hill.

Mallory Walker, president of the Walker-Dunlop mortgage banking firm, said that fewer starts of new commercial buildings are likely in 1981 because of the lack of permanent financing money, and because three-to-five-year construction loans are tied to the prime rate. He said that fairly reasonable rates for construction and long-term financing can be obtained from lenders who are granted a substantial piece of ownership in a project.

Another realty professional said that only solidly structured developers can survive in this high-interest market and fewer marginal developers able to do business.

Commenting on construction loan rates that ranged between 22 and 24 percent a few weeks ago, mortgage banking executive Donald West of Weaver Bros. said that high rates for all real estate loans had been "disastrous for everyone." Small home building firms, for instance, cannot afford to begin new projects, even with no substantial inventory of unsold houses, because of the difficulty of operating with capital that costs more than 20 percent.