Whether working directly with the client or through an agency, a freelancer architectural illustrator may be asked to sign one or both. It's also possible that a client or agency won't require either.
Let's start with an NDA; a Non-Disclosure Agreement. It's common that an agency will ask you to sign an NDA. Often, it means that their client has asked them to sign an NDA and that everyone working on the project understands that the work is confidential.
NDAs is written by attorneys, so that language is dense and confusing. I'll explain the basic gist of an NDA. The document states the date and place of the agreement along with the party's legal names. It ensures that the project is confidential and restricts any disclosure about any part of the project for a particular amount of time. Sounds simple, right? Yes, and no.
You may be asked to sign a Work for Hire agreement. It states that you will not own any of the work reproduced, even work that wasn't used. Work for Hire agreements means that, for whatever fee, you retain no copyright to the work you have done. If you are employed or an architectural illustration freelancer, the law states that, by default, you're working in a Work for Hire agreement. This ensures that when an agency sells your work to a client that it's theirs to sell. If you're working directly with the client, your agreement can be different.
Work for Hire agreements are very restrictive and should not be entered into lightly. As a freelance architectural illustrator, you need to be aware of who owns your work and when copyright transfers. It's common that copyright transfers from the designer to the client or agency when the designer is paid. Architectural Illustrators have contracts that sell limited rights to these images as opposed to the image itself. If you are acting as an architectural Illustrators, you should have two separate agreements stipulating what the agency owns or does not own, at what point, and for how long.
My final recommendation is that if you're in any way confused about a document given to you to sign, ask the agency or client to go through it with you point by point, just to make sure you're absolutely clear what you're agreeing to. Under no circumstances should you sign a document if you're not clear about what specific actions you need to take.