Surely this comes down to the size of the project? Most boat design, I would suggest, is largely developmental rather than totally original. So there is plenty of scope for developing from what already exists. I am not here suggesting plagiarising a specific design, but rather taking a generic boat shape/method of construction as a starting point.
At a very basic level, my first craft was a canvas on frame kayak; the only easily available plan for me at that time, c 1947/8, was a British Scout Kayak. This could be built as a 16'0" long two-seat version, an 11'0" single. I thought thought the 11'0" was too short, and extended the length to 14'0" by moving the centre frames a bit further apart. It worked fine, and enabled me, a very poor schoolboy, to paddle the Thames, and several other rivers very happily. If you really must design your own boat, there are a number of excellent books available which will at least guide you in the right direction. Enjoy yourself! dddesigns.