Hidden costs buyers only discover after possession

Been seeing ads for ATFL villas in Sector 155. Currently in a 2BHK, thinking of upgrading. Everyone talks about the base price, but what about hidden charges? Registration, stamp duty, parking, PLC...it all adds up. Is it worth the price jump? Already have a home loan, so EMI will be a big factor. Also, is resale of villas easy? Any experiences? What did you wish you knew before buying? What hidden costs did you miss?