From Arlington County:
The County Board, responding to what they said is a growing crisis, had been asking developers of residential buildings to set aside voluntarily at least 10 percent of floor space for affordable housing. Board members similarly asked developers of office buildings to volunteer to make substantial monetary contributions.
Friday, Arlington Circuit Judge Joanne F. Alper issued a ruling finding that those requests were in fact requirements, and that as a result, the county had exceeded its legal authority under state law.
For more, see this Reason study.