No. You don’t have to wait until you build the product or prototype before you apply for a patent for your invention. However, you should know that an idea alone is not patentable. The “idea” is the first critical step toward obtaining a patent. However, you need to disclose enough details to satisfy the written description and enablement requirements of the patent code.
The enablement requirement mandates that you describe your invention in such terms that a person of ordinary skill in the art is able to make and use your invention without undue experimentation. You can rely on the skills that a typical practitioner in the field of your invention would have; i.e., if your invention is in the field of furniture, you can rely on standard construction techniques being known and you do not have to disclose such in detail in the application. However, you must describe any elements of your invention that are patentably distinct from the prior art in detail, as, by definition, a person of skill in the art would not be aware of these elements.
The written description requirement requires that your description must describe your invention in sufficient detail that one skilled in the art can reasonably conclude that you have possession of the invention. The detailed description must clearly describe every element of your invention that you intend to recite in the claims of your application.