In the United States, secondary suites are generally referred to as accessory dwelling units or "ADUs". Zoning permissions and laws concerning accessory dwelling units can vary widely by state and municipality. Accessory dwelling units were popular in the early 20th century in the United States. However, ADUs became less common after WWII, when a shift to suburban development occurred and many municipalities banned ADUs through zoning regulations. With increases in the price of housing in many cities and suburbs, increased awareness of the costs of low-density car-oriented development patterns, and an increased need to care for aging Americans, many government entities and advocacy groups have increasingly supported ADUs. Some critics perceive ADUs to be a threat to the character of single-family residential neighborhoods.