diff --git a/packages/core/src/hooks/spatial-grid/spatial-grid-manager.ts b/packages/core/src/hooks/spatial-grid/spatial-grid-manager.ts
index f1a365d0..6861dcd0 100644
--- a/packages/core/src/hooks/spatial-grid/spatial-grid-manager.ts
+++ b/packages/core/src/hooks/spatial-grid/spatial-grid-manager.ts
@@ -404,6 +404,40 @@ export class SpatialGridManager {
     return maxElevation
   }
 
+  /**
+   * Get the slab elevation for a wall by sampling its start, mid, and end points.
+   * Returns the highest slab elevation found, or 0 if none.
+   */
+  getSlabElevationForWall(
+    levelId: string,
+    start: [number, number],
+    end: [number, number],
+  ): number {
+    const slabMap = this.slabsByLevel.get(levelId)
+    if (!slabMap) return 0
+
+    const samples: [number, number][] = [
+      start,
+      [(start[0] + end[0]) / 2, (start[1] + end[1]) / 2],
+      end,
+    ]
+
+    let maxElevation = 0
+    for (const slab of slabMap.values()) {
+      if (slab.polygon.length < 3) continue
+      for (const [sx, sz] of samples) {
+        if (pointInPolygon(sx, sz, slab.polygon)) {
+          const elevation = slab.elevation ?? 0.05
+          if (elevation > maxElevation) {
+            maxElevation = elevation
+          }
+          break // This slab matched, no need to check more sample points
+        }
+      }
+    }
+    return maxElevation
+  }
+
   clearLevel(levelId: string) {
     this.floorGrids.delete(levelId)
     this.wallGrids.delete(levelId)
diff --git a/packages/core/src/systems/slab/slab-system.tsx b/packages/core/src/systems/slab/slab-system.tsx
index 1240ab8a..e62dda56 100644
--- a/packages/core/src/systems/slab/slab-system.tsx
+++ b/packages/core/src/systems/slab/slab-system.tsx
@@ -40,11 +40,65 @@ function updateSlabGeometry(node: SlabNode, mesh: THREE.Mesh) {
   mesh.geometry = newGeo
 }
 
+/** Half of default wall thickness — used to extend slab geometry under walls */
+const SLAB_OUTSET = 0.05
+
+/**
+ * Expand a polygon outward by a uniform distance.
+ * Offsets each edge outward then intersects consecutive offset edges.
+ */
+function outsetPolygon(polygon: Array<[number, number]>, amount: number): Array<[number, number]> {
+  const n = polygon.length
+  if (n < 3) return polygon
+
+  // Determine winding via signed area
+  let area2 = 0
+  for (let i = 0; i < n; i++) {
+    const j = (i + 1) % n
+    area2 += polygon[i]![0] * polygon[j]![1] - polygon[j]![0] * polygon[i]![1]
+  }
+  const s = area2 >= 0 ? 1 : -1
+
+  // Offset each edge outward by amount
+  const offEdges: Array<[number, number, number, number]> = []
+  for (let i = 0; i < n; i++) {
+    const j = (i + 1) % n
+    const dx = polygon[j]![0] - polygon[i]![0]
+    const dz = polygon[j]![1] - polygon[i]![1]
+    const len = Math.sqrt(dx * dx + dz * dz)
+    if (len < 1e-9) {
+      offEdges.push([polygon[i]![0], polygon[i]![1], dx, dz])
+      continue
+    }
+    const nx = (s * dz / len) * amount
+    const nz = (s * -dx / len) * amount
+    offEdges.push([polygon[i]![0] + nx, polygon[i]![1] + nz, dx, dz])
+  }
+
+  // Intersect consecutive offset edges to get new vertices
+  const result: Array<[number, number]> = []
+  for (let i = 0; i < n; i++) {
+    const j = (i + 1) % n
+    const [ax, az, adx, adz] = offEdges[i]!
+    const [bx, bz, bdx, bdz] = offEdges[j]!
+    const denom = adx * bdz - adz * bdx
+    if (Math.abs(denom) < 1e-9) {
+      // Parallel edges — use offset endpoint
+      result.push([ax + adx, az + adz])
+    } else {
+      const t = ((bx - ax) * bdz - (bz - az) * bdx) / denom
+      result.push([ax + t * adx, az + t * adz])
+    }
+  }
+
+  return result
+}
+
 /**
  * Generates extruded slab geometry from polygon
  */
 export function generateSlabGeometry(slabNode: SlabNode): THREE.BufferGeometry {
-  const polygon = slabNode.polygon
+  const polygon = outsetPolygon(slabNode.polygon, SLAB_OUTSET)
   const elevation = slabNode.elevation ?? 0.05
 
   if (polygon.length < 3) {
diff --git a/packages/core/src/systems/wall/wall-system.tsx b/packages/core/src/systems/wall/wall-system.tsx
index 1c207182..264788ae 100644
--- a/packages/core/src/systems/wall/wall-system.tsx
+++ b/packages/core/src/systems/wall/wall-system.tsx
@@ -2,6 +2,8 @@ import { useFrame } from '@react-three/fiber'
 import * as THREE from 'three'
 import { Brush, Evaluator, SUBTRACTION } from 'three-bvh-csg'
 import { sceneRegistry } from '../../hooks/scene-registry/scene-registry'
+import { spatialGridManager } from '../../hooks/spatial-grid/spatial-grid-manager'
+import { resolveLevelId } from '../../hooks/spatial-grid/spatial-grid-sync'
 import type { AnyNode, AnyNodeId, WallNode } from '../../schema'
 import useScene from '../../store/use-scene'
 import {
@@ -95,18 +97,22 @@ function getLevelWalls(levelId: string): WallNode[] {
  * Updates the geometry for a single wall
  */
 function updateWallGeometry(wallId: string, miterData: WallMiterData) {
-  const node = useScene.getState().nodes[wallId as WallNode['id']]
+  const nodes = useScene.getState().nodes
+  const node = nodes[wallId as WallNode['id']]
   if (!node || node.type !== 'wall') return
 
   const mesh = sceneRegistry.nodes.get(wallId) as THREE.Mesh
   if (!mesh) return
 
+  const levelId = resolveLevelId(node, nodes)
+  const slabElevation = spatialGridManager.getSlabElevationForWall(levelId, node.start, node.end)
+
   const childrenIds = node.children || []
   const childrenNodes = childrenIds
-    .map((childId) => useScene.getState().nodes[childId])
+    .map((childId) => nodes[childId])
     .filter((n): n is AnyNode => n !== undefined)
 
-  const newGeo = generateExtrudedWall(node, childrenNodes, miterData)
+  const newGeo = generateExtrudedWall(node, childrenNodes, miterData, slabElevation)
 
   mesh.geometry.dispose()
   mesh.geometry = newGeo
@@ -114,7 +120,7 @@ function updateWallGeometry(wallId: string, miterData: WallMiterData) {
   // Update collision mesh
   const collisionMesh = mesh.getObjectByName('collision-mesh') as THREE.Mesh
   if (collisionMesh) {
-    const collisionGeo = generateExtrudedWall(node, [], miterData)
+    const collisionGeo = generateExtrudedWall(node, [], miterData, slabElevation)
     collisionMesh.geometry.dispose()
     collisionMesh.geometry = collisionGeo
   }
@@ -134,12 +140,13 @@ export function generateExtrudedWall(
   wallNode: WallNode,
   childrenNodes: AnyNode[],
   miterData: WallMiterData,
+  slabElevation = 0,
 ) {
   const { junctionData } = miterData
 
   const wallStart: Point2D = { x: wallNode.start[0], y: wallNode.start[1] }
   const wallEnd: Point2D = { x: wallNode.end[0], y: wallNode.end[1] }
-  const height = wallNode.height ?? 2.5
+  const height = (wallNode.height ?? 2.5) - slabElevation
   const thickness = wallNode.thickness ?? 0.1
   const halfT = thickness / 2
 
@@ -232,6 +239,9 @@ export function generateExtrudedWall(
 
   // Rotate so extrusion direction (Z) becomes height direction (Y)
   geometry.rotateX(-Math.PI / 2)
+  if (slabElevation > 0) {
+    geometry.translate(0, slabElevation, 0)
+  }
   geometry.computeVertexNormals()
 
   // Apply CSG subtraction for cutouts (doors/windows)