With hardware you can check your designs against the laws of physics. With software there usually aren't any such laws. Being a guy who has actually written a 1300 page requirements document in my career as well as large books full of designs, my experience has been that the only thing you can test them against is code. Usually you are wrong :-). It's much better to start off with tiny pieces and make sure to write a lot of code along the way. Exactly how much upfront analysis you need before you start writing code is a judgement call, but I would say that it's almost always safer to err on the side of writing code too soon.