Hi Flo, not sure about a design planner. However, we bought a 30ft narrowboat last year after looking at dozens of the same size with very varied layouts. It does depend on what you want but ours has a fixed double, then a shower/loo, then a small kitchen and open plan (dining/lounge) with doors opening on the front of the boat. They used every available bit of space and if I was designing from scratch I couldn't have done a better job. If you let me know your email I've got plenty of interior pictures.
regards,