The Interaction Design Process

Clearly it is important that what is designed is feasible. It's utterly pointless to design something that can't be built. But it's more complex than just is it feasible. If some part of the system has to perform complex calculations, or do a lengthy database lookup then that is a constraint on the design of a system. The way you signal to a user that their request may take a minute is very different from the way you deal with delays of a second or so. These issues need to be understood early in the design process to ensure that the design matches the technical capabilities of the systems behind it.

Development costs money of course, and unless you're in the fortunate position of having plenty of that (and who can say that at the moment?) you need to ensure you're spending it wisely. If you're aiming to differentiate your business by offering the very best user experience then that will often increase development costs. If you are forced to be somewhat more pragmatic then make sure your designer knows what you can and can't realistically achieve. To do this your designer is going to have to spend some time with your developers gauging how much effort different design approaches would cost.