The Washington metropolitan area is one of the few parts of the country where residential land prices remain high despite a national trend to lower prices, according to a new study. The study says the slumping prices indicate that real estate is fast becoming an "overrated investment."

Except for the Washington area and the Sun Belt sections of the South, the Southwest and California, the price of land for home building was found to be at its lowest level in 2 1/2 years.

The Homer Hoyt Institute, a Washington-based nonprofit organization that analyzes development problems and real estate opportunities, conducted a county-by-county review of land prices by the square foot. The study found that the District of Columbia, "which is not really a county but approximates one," showed the highest cost in the United States -- $11.39 a square foot. It was followed by Honolulu at $10.59 and San Diego with $4.25. Fairfax County ranked 14th with $2.19 and Montgomery County was 16th with $2.18.

An accompanying review of average costs of finished lots found that D.C. ranked third last year, with an average cost of $23,732, after Hawaii, where the average cost was $62,516, and California, at $30,853. Maryland ranked fifth at $20,408, and Virginia was sixth at $19,535. A finished lot is a piece of property already serviced by water and sewer lines on which a house is under construction.

Michael Sumichrast, treasure of the Hoyt group as well as chief economist of the National Association of Home Builders, cautioned that comparing the District with states or even counties was misleading, since prices of land in almost any city are bound to be considerably higher than suburban or rural property.

Bearing this in mind, Sumicharast noted that the District ranked high in yet another measurement -- the percentage that the cost of finished lots represented in the total value of a house. Hawaii, where a severe shortage of usable land has long kept prices sky-high, ranked first in this scale, with 44 percent; D.C. was second with 31 percent; Maryland was eighth with 32.2 percent while Virginia ranked 14th with 22.1 percent.

Generally, Sumichrast said, the Washington metropolitan area escaped the harsher declines found elsewhere in the country for three reasons: "It is part of the fast-growing South.There is a continuing influx of labor, trade associations and other employers. And there is still room for expansion."

The nationwide decline, following inflation-fueled, double-digit growth during the 1970s, parallels the drop in new housing construction in most parts of the country. As fewer houses are being bought, largely as a result of unaffordable mortgage interest rates, builders are cutting back on parcels of land they're buying for new projects.

"With record-high mortgage rates and housing in a slump, it is highly likely that land prices will continue to be depressed throughout 1981 and into 1982," the study found.

"In the last few months," it continued, "land prices changed dramatically. The hefty increases are no longer there, and in many areas prices actually declinded. In the near future (the next 12 to 24 months), land prices are not likely to go anywhere, as housing suffers with slow sales, bankruptcies and difficulty in trying to move the houses as well as land.

"With inflation clearly coming down, land is fast becoming an over-rated investment. So is real estate in general, vis-a-vis other types of investment . . . The longer-term prognosis suggests a further decline. With inflation expected to be lower, prices simply cannot keep up the pace."

The study's findings, which center on a new land price index developed by two economics professors at Temple University in Philadelphia, are reported in the first issue of "Land Trend," a monthly bulletin published by the Hoyt group. The raw data for the index was supplied by local builders around the country through the Home Owner Warranty Corp., which runs a 10-year insurance program for new houses.

Paul Rappaport, who developed the land price index along with Michael Goetz, said they began to notice sluggishness in the residential land market in the second half of 1980. "There was some growth nationwide," Rappaport said, "but at a reduced rate. Parts of the Southwest and South had good growth, but the Northeast and the Mid Atlantic states did not."

In the North Central section of the nation, with Chicago at its center, land prices actually depreciated, by 3 percent, the study found. Nationwide, the cost of land appreciated just 2.9 percent in the first quarter of this year and 4.6 percent in the final quarter of last year.

By comparison, nationwide prices rose by as much as 18.1 percent annual rate during the first three quarters of 1980 and, before that, never less than 10 percent in any two consecutive quarters since 1977.

In the Southern region, which includes the District, Maryland and Virginia, prices rose at the rate of 9.7 percent in the first three months of 1981, following a 5.1 percent rise in the final quarter of last year.

In the Northeast, after a 4.1 percent growth rate in the final quarter of last year, the study show an extraordinary 20.5 percent rise for the first quarter of this year. However, Rappaport said, this was an "aberration," caused by "a couple of sales of exceptionally valuable -- perhaps seafront -- property."