Changing felt roof to slate or tile

Hi, I'm looking at houses at the minute with my son and have come across a 3 bedroom bungalow that I'm interested in, with a slanted roof that is felt (pitched roof). How much roughly would it cost to change it to tiles or slate; is it a big job? I would be looking at rendering the outside as well as it appears to be pebble-dashed. (Just looking an approximate ballpark figure to factor in should I put an offer in). Thanks in advance