For years in Fairfax County, the job of making sure developers held up their end of the bargain on public improvements came down to a single staff worker. And while scores of builders reshaped the county, promising roads, sidewalks, sewers, playgrounds and other amenities, the government struggled to enforce the completion of those projects.

Now, there are more than 600 projects in “default,” which means developers failed to complete the improvements by an agreed-upon time, according to a recent report by Fairfax County auditors.