Your question is so vague it's almost not possible to answer!
Buildings need maintenance practically every year. What and when the need things depends on were and how they are built. Buildings vary from practically mantainance free to a whole lot of mantainance. If everything is left for 15 years there is bound to be a lot of cleaning and repairs!
Fix anything worn, broken or otherwise in general disrepair! Clean and prep and repaint anything that need paint. Check for bad roofing and driveway conditions, Most likely it was time to refinance depending on the rate 15 years ago. check for leaks and general condition of plumbing and windows. You probably would have to clean the windows. After 15 years of no mantainance you may have squatters living there!