A contract arises when one party makes an offer and the other party accepts it, even if they haven't signed anything or the agreement was made verbally. If possible, you should still get the details of the agreement in writing.
For work worth $10,000 or more, you must have a written contract with a registered builder. This is known as a major domestic building contract. For work that is under $10,000, it is still recommended that you use a written contract.
You can engage a professional to manage a building contract or building works on your behalf, such as an architect, building consultant or project manager. They can ensure the building work meets the standards and quality stated in the contract. See Roles and responsibilities to understand the types of professionals that may be involved in your project.