This can often be termed as the Brief and should be worked up alongside your Architectural Designer.
However, it is advisable to carefully consider your requirements prior to the initial meeting with the designer so the meeting can be as constructive as possible. Knowing what you want is extremely important.
This initial step is essential in you carefully considering your needs and future aspirations. As a family, are you considering expansion? Are you considering starting your own business and will you require office space? How flexible should the internal layout be? Should the design of the proposal be future proofed for resale later?