The client may, or may not, solicit the designer for ideas on site usage. Getting feedback from the architect regarding site usage is a good idea, as the architect generally has the benefit of knowing the local planning commission, local ordinances, economy, micro-culture - and has a good feel for what can be accomplished within the proposed budget.
Architects begin by acquiring topo/property maps and legal descriptions of the site, followed by zoning and urban planning documents. Generally, they will also use soil engineers to ascertain the make-up of the site. This information is essential in deciding what kind of foundation will be required, what utility/municipal easments and restrictions apply, sun/wind/drainage issues - all of which play a role in determining the form-constraints of the design.
A requirements document is then compiled, and if warranted by the scope of the project - volume, space adjacency and facilities matrices are modeled. It is amazing how much space parking, restrooms and mechanical facilities (heating/cooling) can take up.
Only after these (and possibly many other) steps are completed, is the designer ready to begin concept drawings. Some clients will give the designer considerable freedom and control at this stage - others do not. In any case, the alert designer must be prepared for the inevitable: the client abruptly changing conceptual direction or downgrading the budget at the last moment.
My task was to: design the 3rd building, do the interior of the bank, and to tie all three buildings together into a unified whole. The third, central building was to be a mixed-use residential and light commercial building.