If your invention requires further development, either before or after filing a patent application, then it is prudent to have a written agreement with the person who is carrying out the development work. Unless there is clear written agreement before the work is done, development work on an invention is a frequent cause of disputes as to who owns the invention as developed. For a sample agreement, download our Agreement to Develop Invention (PDF, 48K).
The basis on which the development work is done may be that you, the inventor, own all of the developed invention, or you may prefer that the developer enters into some sort of joint ownership agreement of the invention and owns a proportion of the developed invention; in this case, the developer usually would carry out the development work either at no cost or at reduced cost. However, joint ownership of patents can cause difficulties in practice, and it is sensible to check with a patent attorney first if you are considering a joint venture agreement of this type.