The for-sale inventory is shifting away from completed homes toward those under construction or not yet started, according to Jed Kolko, Trulia’s chief economist. 

According to data from the U.S. Census Bureau this week, new-home inventory was up 5.5% year-over-year in March. However, the inventory of completed new homes actually dropped 16% over the same period. 

Only 27% of new homes for sale are completed, Trulia (TRLA) reported. With a tightening housing market, the completed share of new-home inventory will continue shrinking as buyers snap up completed homes more quickly and builders lock in buyers earlier. Additionally, new starts are adding to the for-sale inventory that is not yet complete. 

Completed new-home inventory dropped from 26% in early 2000 to nearly 21% at the top of the bubble in 2006. Once demand began drying up, the completed share jumped to 49% in January 2009. In the past four years, it has dropped back to 27% as of March 2013.

“It’s more evidence that builders’ biggest challenge is keeping up with demand. For buyers looking for new construction, they’ll need imagination and patience: imagination in order to picture what the unfinished home they’re bidding on might look like, and patience because they won’t be moving into that unfinished home tomorrow,” writes Kolko.