help ?? what would you do ? do you think this is a nice idea ??

I currently live at my grandparents with my boyfriend instead of getting a house of our own my grandparents suggested we build an extension on the garage and get the loft converted for us as she is leaving the property to us when she dies and she thinks its a nice idea to pay for the extension and the loft conversion so when we want to sell the house we will make a bit of profit on it ! as she dosnt want us to move out and get into debt so she suggested we get a loan or a mortgage to do the home improvments so we had a living room on the garage, leading into my bedroom then going up into the loft for either an extra bedroom or whateva ?do you think this is a nice idea ?? also would we have to get a loan or could we get a mortgage to do the home improvements as its my grandparents house ?? obvoisly i will see the deeds first to make sure my name is on even though they wudnt lie give your opinions ? also the house is worth £200,000 and its a 3 bedroom and if we add 2 more rooms on we would definatly make a lot more profit when we come to sell the house ! they are due to retire soon and me and my boyfriend will be funding the project ! xx to patrona i do trust them very much ! ! i needed an opionion about doing this or should i buy my own place ! !
Answer
I like that idea and you have nothing to lose