A lightweight platform, using the existing ceiling joists, might be fairly simple. But you'd need to leave any cross beams supporting the roof structure in place. You'd also need to put up insulation around the roof, and board over it. And it wouldn't be strong enough to support anything but the lightest of use (i.e. no furniture). I'd really question the usefulness and financial feasibility of something like this. If you want the new floor to be able to support furniture and other heavy usage, you'd need to insert new steel (or thick wood) joists. If you want natural light, you'd need to install a velux window. By the time you've done all this, you basically have a loft conversion.