Do you have an existing garage, a parking pad or an undeveloped part of the yard? Those are different starting points. Make a note of what is already on site and what you would like to retain. An existing slab should not be assumed suitable for a new structure without an assessment.
Record whether access is from a rear lane or the street, where vehicles will turn and how construction materials could reach the proposed location. Bring a property survey or real property report when available; a rough sketch is useful for discussion but is not confirmation of property boundaries.