If you've wondered why the prices of new homes have continued to spiral upward despite relatively low inflation in the national economy overall, look to one key factor: land values.

A soon-to-be-released study of residential land prices in 30 metropolitan housing markets reveals that the average value of undeveloped residential acreage has jumped at twice the pace of inflation over the past 60 months.

In some high-growth markets, land values for housing have risen by more than 200 percent since 1980, according to the Urban Land Institute, which is based in the District. The national average price increase in residential land during those 60 months was 51.3 percent per acre. The consumer price index, by comparison, rose 26 percent in the same period.

Cities with strong, fast-expanding high-tech employment bases have seen land values go off the charts in some cases, according to the twice-per-decade study. Raleigh, N.C., topped the nation with a 213.7 percent five-year growth in the value of raw residential acreage. Investors who bought an average-priced parcel of undeveloped land in metropolitan Raleigh for $3,250 an acre in 1975 could sell it for $20,000 an acre last month (an appreciation in excess of 500 percent), according to ULI's research.

The price of raw land in metropolitan Hartford, Conn., more than doubled in the past 60 months -- up by 108 percent, from $7,250 an acre in 1980 to $15,000 in 1985. Other big gainers on the national land-inflation charts included Boulder, Colo. (up by 100 percent in five years); Phoenix and Charlotte, N.C. (up 91 percent); metropolitan Boston (up 85 percent), and Atlanta (up 78 percent).

Land-price inflation in some high-tech, high-gainer areas of the 1970s slowed over the past few years, but still pushed housing prices hard, according to ULI's study. San Jose, the microchip mecca of the West Coast, registered a mere 29 percent growth rate in residential land values between 1980 and 1985. When added to the 582 percent average growth in per-acre values the prior five years, however, San Jose jumps into first place nationwide for sustained land inflation during the decade.

A parcel of undeveloped San Jose land that you could have purchased for $27,500 an acre in 1975 would sell for $240,000 today, according to J. Thomas Black, the ULI vice president who headed the national survey team. A 10,000-square-foot building lot in San Jose that sold for $40,000 in 1980 sells for an average $70,000 today.

"Clearly, inflation is far from dead," Black said. "We should have all invested in land five years ago in places like Raleigh, San Jose, Boston and Charlotte."

But Black emphasized that high gains in land values "are not necessarily good news," especially from the point of view of new-home purchasers, home builders, employers and future economic growth for the metropolitan areas. He noted that rapid run-ups in land prices can indicate that local governmental policies are making prime land more difficult to develop for housing through zoning practices, high fees and other growth-control restrictions. Metropolitan Boston's 90 percent surge in the cost of a typical 10,000-square-foot home lot since 1980 is partly attributable to excessive local governmental restrictions imposed on builders, Black said.

Boston's price of $45,000 for an average building lot in ULI's study catapulted the area to second most costly among the 30 major cities studied. That's a big jump from its relative position just five years ago.

Such high land costs, in turn, inevitably will "hold down growth in the coming years," according to Black. Employers who otherwise might have expanded in the area will shift to less-costly cities in the Northeast. Employes who otherwise might have been transferred to Boston will end up elsewhere, the direct result of superheated land inflation.

Today's hot land markets, in fact, almost certainly are fated to cool off in the coming five years. Portland, Ore., and Miami, for instance, were among the top gainers in ULI's last study, five years ago. Portland had registered a 120 percent average gain in value per acre; and Miami, ll3 percent. Over the past 60 months, Portland lots in the ULI study registered no increase in value, and Miami parcels gained just 20 percent.