Drainage fall is an intentional change in level that directs water across a surface. It is relevant to roofs, paving and some wet areas. The appropriate arrangement depends on the complete detail and selected system.
How this term relates to your project
For an extension, show where water travels and identify its next connection. A slope without a suitable outlet does not complete the design. On roofs, consider interruptions such as rooflights and upstands. Outside, coordinate ground level, threshold and paving.
Do not apply an isolated rule or generic percentage without assessing the specific use. Ask the installer to explain the assumptions and how the result will be checked. Keep level references on the same drawing so roofing and landscaping contractors do not unknowingly use different datum points. The visible direction of a surface is only one part of a functioning drainage route.