Adaptive reuse converts existing structures to new purposes. Common conversions: warehouse→residential/office, church→home/event space, factory→loft/brewery, school→apartments, bank→restaurant, firehouse→home, barn→event venue. Benefits: embodied carbon savings (50-75% vs new construction), historic character, neighborhood preservation, faster permitting in some jurisdictions, tax credits (Federal Historic Preservation Tax Credit = 20% for qualified rehab). Challenges: code compliance (existing building code vs new construction), ADA accessibility, hazmat (asbestos, lead, PCBs), structural capacity for new use, hidden conditions. Cost: $150-$400/sf depending on existing conditions.