Washington ? Construction of new homes and apartments rose 4.7 percent in January to the highest level in more than two decades as low mortgage rates continued to power the nation’s housing industry.

The Commerce Department reported Wednesday that builders began construction on 2.16 million units at a seasonally adjusted annual rate last month, up from a rate of 2.06 million units in December.

The January increase caught economists by surprise. They had been forecasting a decline of around 3.7 percent, reflecting rain in the West and winter storms in the East, which had been expected to hold down construction activity.

Instead, builders, enthused by continued low mortgage rates, broke ground on the largest number of new homes and apartments since February 1984 when construction starts hit an annual rate of 2.26 million units.

“With cheap mortgages the perfect aphrodisiac, the American love affair with housing continues and developers are more than willing to comply with their customers wishes,” said Joel Naroff, chief economist at Naroff Economic Advisors. “Low mortgage rates trump everything else as the housing market continues to boom along.”

In Lawrence, builders hungry to build more houses have been hampered by a lack of available lots. Builders took out five permits for new single-family homes in January, the lowest total for the month since 1997. The city issued no permits for duplexes or apartments last month.

The report on housing starts showed that construction of single-family homes rose 2.7 percent to 1.76 million units, an all-time high, while construction of apartments was up a sharp 14 percent to 399,000 units.