There are many companies selling garden buildings, log cabins, bespoke garden offices and the like. A glance through their brochures, which are usually provided for free, or website pages should give you a good idea of a wide variety of designs. Once you know the size, shape and finish of building you want, you can get a suitably qualified draughtsman to draw it up for you. You must bear in mind, though, that because you want an upper floor, you will inevitably exceed the height limit for permitted development, which means you will need planning permission. You only need quite simple drawings for this initial step of the planning phase, then you can add in all the construction details once permission is granted.