Remodeling Magazine's payback report

Based on national averages, a study from Remodeling Magazine found that exterior upgrades tend to have a greater return on the investment than interior upgrades. According to the study, the highest percentage of costs were recouped from upscale siding replacement (88%), a wood deck addition (85%), wood window replacement (81%), and a minor kitchen remodel (83%). Some of the projects that received the lowest returns are home office remodels (57%), installing a back-up generator (58%), and adding a mid-range sunroom (59%). The value of the renovations dropped since the 2006 survey because of rising remodeling costs and a slower housing market.

http://www.realtor.org/rmomag.NSF/pages/2007CostvsValue?OpenDocument