Yes, a lot! According to a recent survey carried out amongst estate agents in London, between 15 to 25% of the value of your flat! In our experience, you will get back every pound you spend in additional value, and, if your apartment has no outdoor space, probably as much as a further 50p for every pound you spend. But you should view a roof terrace as primarily bringing you lifestyle benefits rather than as an investment - more space, outdoor space, better insulation for both summer and winter, year-round noise reduction, and a 70% extension to the life of the roof covering!