The number of homes listed for sale rose 2.9% in January, the first increase in 18 months, according to Zip Realty, the online and local real estate brokerage firm.

ZIP's Housing Inventory Index, a survey of Multiple Listing Service (MLS) listed homes in 27 major U.S. housing markets, showed a increase of 15,000 homes on the market last month compared to December. Still, the number of home listings year-over-year remains down 22.3% (-163,000 homes), with the combined number of MLS-listed single family homes and condos within the 27 major U.S. markets surveyed in January totaled 567,265, up from 551,447 in December. The average median list price of $258,634 was relatively flat in January, down 1% (or $2,521).

ZipRealty president and CEO Patrick Lashinsky attributed the increase to the home buyer tax credit that is slated to expire April 30 for sales and June 30 for closings. "Serious sellers need to list their home now, rather than wait for the spring, to capitalize on buyers looking to take advantage of the tax credit extension," he said. "While the number of homes for sale is starting to increase, we are still seeing some markets with a shortage of homes for sale."

Markets in California showed significant jumps in inventory in January with month-over-month increases of 10.1% in the San Francisco Bay Area, 8% in Orange County, 4.5% in Los Angeles, and 6.5% in San Diego.

Among individual markets, Austin rose 5.1%; Baltimore dropped 1.9%; Boston rose 4.8%; Charlotte fell 3.5%; Chicago rose 1.5%; Dallas-Ft. Worth rose 3.3%; Denver rose 6.7%; Houston rose 6.0%; Jacksonville rose 0.9%; Las Vegas rose 5.7%; Miami fell 0.5%; Minneapolis-St. Paul rose 2.8%; Norfolk/Virginia Beach rose 1.5%; Orlando rose 0.3%; Philadelphia rose 3.6%; Phoenix rose 2.3%; Raleigh/Durham rose 4.2%; Richmond rose 1.5%; Salt Lake City rose 2.3%; Sacramento rose 1.6%; Seattle rose 2.6%; Tucson rose 5.4%; and Washington, D.C. rose 0.9%.

Learn more about markets featured in this article: Los Angeles, CA, San Francisco, CA.