Feb. 20 (Bloomberg) -- Demand for U.S. architectural services fell in January for the first time in four months as developers concerned about a recession cut spending, the American Institute of Architects said.

The Architecture Billings Index declined to 50.7 last month from 55 in December, the Washington-based institute said today in a statement. That was the lowest score since July 2006, when it was also 50.7. Any score above 50 indicates an increase in billings from the previous month.

The drop may signal a ``sustained'' decline in demand from developers of warehouses, offices and apartment buildings as the economy slows, the institute said. Weakness in the U.S. housing market is spreading to other parts of the economy and prompting economists to strengthen their forecasts of a recession. A Bloomberg News survey taken earlier this month showed chances of a recession were now even, compared with 40 percent in January.

``I think the economy has taken a turn for the worse in the last couple months, and projects that made sense last fall may make a lot less sense now,'' Kermit Baker, chief economist of the American Institute of Architects, said in an interview. ``We're going to see several more months of weakness in design billings. I'm guessing this is going to be the last positive month we're going to see for the next few.''

Architecture billings declined in the Midwest, which had a score of 49.3. Billings growth was the highest in the Northeast with a score of 63, followed by the South at 53.9 and the West at 51.3, the institute said.

The index is based on a survey of firms owned by members of the American Institute of Architects. Participants are asked each month whether billings increased, decreased or were unchanged in the month just ended. The institute began conducting the survey in 1995.

To contact the reporter on this story: Daniel Taub in Los Angeles at dtaub@bloomberg.net.

Last Updated: February 20, 2008 00:00 EST