I’m asked this question exceptionally frequently. In fact, almost every client who hasn’t yet built their idea out into at least a first prototype wants to know if they need to take this step. The answer is no, usually.
There is certainly no specific law that says anything like “you must create a prototype prior to filing a patent application,” or any black-and-white rule that requires something be built before hand. Like all things with law, the requirement is much more in the gray.
Before a patent application can be filed, the invention must have actually been “invented” in a legal sense. Patent law defines the act of inventing as the conception of an idea followed by the reduction to practice of that idea. “Reduction to practice” can be achieved by actually building your idea out, or by being able to describe it with sufficient detail such that you can show you were in possession of the idea, and so that someone with ordinary skill in the field of invention would know how to make your idea and how to use your idea. This test has no clear boundaries; the need to build a prototype is going to depend on the level of complexity of the invention.
For example, if you’ve come up with a new kind of shoelace, there probably isn’t a great of complexity in the design. It likely isn’t going to require a ton of time to describe what the structure and materials of the shoelace are. The amount of time needed to describe an invention is generally a proxy for its level of complexity. With a shoelace, if you can describe it to your patent attorney within a few hours, it probably doesn’t need to be built out.
If, on the other hand, you’ve invented a new kind of heart medicine, then a build is almost certainly required. Medication can take a huge amount of research, experimentation, trial-and-error, and money to perfect. In order to be able to describe the chemical formula of the medication, you will have to figure out what that chemical formula is, by first developing the drug.
So, the need to build a prototype depends to a large extent on the complexity of the invention. However, it is usually beneficial to build one anyway. Prototypes help inventors flush out design problems and improve the invention in ways that may present commercial significance or marketing advantages. Prototypes also can make the patent application process much more easier and straightforward. With a physical prototype, the drawings can be prepared very quickly and with a much slighter chance of errors, and the patent attorney can draft the application with much more clarity and precision because he truly knows what every part of the invention is.