Legal Basement Classification on Sloped Sites —
Calculating the Weighted Average Ground Level

A Ground-Level Retailer to One Side, Completely Underground to the Other

When observing structures designed on sloped terrains, it is common to see a storefront that operates as an accessible first-floor commercial retail shop from the main street, but becomes completely subterranean when viewed from the back. Is this story categorized as an above-ground level or a basement? The answer dictates financial feasibility, because **if a level secures formal basement status, its total floor area is fully exempted from Floor Area Ratio ($FAR$) constraints.**

The classification mechanism is anchored in the Building Act. If the mathematical average height from the floor plate to the adjacent ground boundary corresponds to at least half ($1/2$) of that story's total height, it is legally validated as a basement. For a standard 3m ceiling layout, an average buried profile of 1.5m or more is required. The challenge on a slope is that the adjoining ground profile forms a variable line, which introduces the weighted average methodology.

Designing the Subterranean Blueprint — The Shindae Case

What makes this structural regulation immensely powerful is that the baseline grade line is not a rigid fixed constraint, but rather a variable parameter manageable through deliberate design orchestration. Our planning proposal for the Shindae-jigu officetel development in Yongin highlights this strategy. By anchoring the building framework against the higher elevations of the plot, we shaped a highly advantageous weighted average line, validating the commercial retail envelope as a legal basement.