It is tempting to start with a floor plan — from a book, a friend's home, or a saved photo. But the lot usually has opinions, and they are cheaper to accommodate on paper before design commitments are made.
What the site decides
Orientation: sun path and prevailing wind shape room placement, shading, and openings more than preference does.
Access and approach: where you enter the lot determines the sequence of arrival, garage, and service areas.
Terrain: slopes suggest stepping the house with the land rather than against it — often producing better architecture and lower earthwork cost.
Setbacks and restrictions: subdivision rules can fix buildable area, height, and even style before you start.
Drainage and views: both are easier to design with than to retrofit.
The practical sequence
Study the lot, then the brief, then the plan. A feasibility-minded site review before design locks in is one of the least expensive, highest-value steps in a custom home project.