I did my own designs after lots of research and careful mentoring by Dee Williams and the PAD (Portland Alternative Dwellings) crew via their workshops and by hiring them as consultants.
If you are thinking about creating your own designs, or modifying ones you’ve purchased, I highly recommend getting good guidance from experts. Take workshops to know what you’re getting into. Hire professionals to make sure your house will be structurally safe and sound, including proper framing and tie-downs; safe electrical, propane and plumbing systems; and appropriate insulation approaches for your climate so that you don’t get mold and rot. Be smart and don’t risk your health and safety or that of others!
These are an odd assortment of posts written as I went through my design process (which is still evolving):
Designing viscerally (posted on TinyHomes.com)
Tassajara, twigs & ten tiny thoughts (includes advice for those about to build)