Adding central air conditioning to an existing forced-air heating system in a 2,000-square-foot house averages $3,500-$4,000, and can be done by two technicians in 2-3 days, with little or no change to the existing ducting; if ducts must be added, the work time doubles, as does the cost to $7,000-$8,000 or more, according to ThisOldHouse.com .