In the sciences this is not generally possible. Before funding is awarded research projects are assessed by other academics to judge whether they are scientifically valid and are suitable for a PhD student (i.e. is there a reasonable chance that the student will get any results in three years). There are exceptions, a few departments do ask applicants to submit research proposals, based on the research currently taking place in their department. If you want to pay for the PhD yourself then it's a different story. You'll need to find a research group that has expertise in your area. Your potential supervisor will be quick to let you know whether your project is realistic. If you are funding yourself you'll need to cost things like trips to conferences, field trips, etc.