For a new garage, start with the intended location, vehicle access and the space that must remain usable around the structure. For a replacement, record the existing building and slab, what you hope to keep and what may need to be removed. Treat reuse as a question for assessment rather than a guaranteed saving.
Note nearby structures, visible service connections and any documents describing the property. Do not guess boundaries or assume a previous garage establishes approval for a larger replacement. The new proposal needs its own review.