Because your website will likely contain a lot of company or brand assets that you clearly own, it is unlikely that a web designer could successfully claim rights to the work they do for you. However, to be safe, you can upgrade your project with an IP agreement that you'll require your freelancer to sign at the start of the project. This ensures that all rights stay with you. You can also go for another upgrade and ask the freelancer to sign a Non-Disclosure Agreement (NDA), which is an agreement that the freelancer agrees not to discuss or disclose the details of the project to others.