When you hire a deck builder what will guarantee that all goes according to plan? The answer is that you will possess a legal contract that can be used as proof of the agreement. If drafted properly, this piece of paper will validate your agreement as legitimate and binding under the laws of the land.
In most cases this is as simple as evidence that one party makes an offer and the other accepts it. The contract will further act as a written explanation of the detailed responsibilities of both parties participating in the project. This leaves little to interpretation and should prevent most disputes from ever occurring. Both sides must understand and sign the document to execute the contract.
You will want to make sure that all the necessary information is included in one completed document. Using a collection of sketches and notes in place of a formal contract is disorganized and unprofessional. It can also be confusing, contradictory, and ineffective.