diff --git a/packages/core/src/schema/nodes/level.test.ts b/packages/core/src/schema/nodes/level.test.ts
index 839be780d..780d3e3ee 100644
--- a/packages/core/src/schema/nodes/level.test.ts
+++ b/packages/core/src/schema/nodes/level.test.ts
@@ -11,6 +11,20 @@ import { PipeSegmentNode } from './pipe-segment'
 import { PipeTrapNode } from './pipe-trap'
 
 describe('LevelNode', () => {
+  test('defaults baseElevation to 0', () => {
+    expect(LevelNode.parse({ level: 0, name: 'Ground' }).baseElevation).toBe(0)
+  })
+
+  test('accepts a custom baseElevation', () => {
+    expect(
+      LevelNode.parse({
+        baseElevation: 1.25,
+        level: 1,
+        name: 'Split level',
+      }).baseElevation,
+    ).toBe(1.25)
+  })
+
   test('accepts every level-hosted MEP node ID', () => {
     const nodes = [
       DuctSegmentNode.parse({
diff --git a/packages/core/src/schema/nodes/level.ts b/packages/core/src/schema/nodes/level.ts
index ef4de43ae..4fb6f86f9 100644
--- a/packages/core/src/schema/nodes/level.ts
+++ b/packages/core/src/schema/nodes/level.ts
@@ -59,11 +59,16 @@ export const LevelNode = BaseNode.extend({
     .default([]),
   // Specific props
   level: z.number().default(0),
+  baseElevation: z
+    .number()
+    .default(0)
+    .describe("Additive Y offset in meters applied above this level's computed stack position."),
 }).describe(
   dedent`
   Level node - used to represent a level in the building
   - children: array of architectural, equipment, and MEP distribution nodes
   - level: level number
+  - baseElevation: additive Y offset in meters above the computed stack position
   `,
 )
 
diff --git a/packages/editor/src/components/ui/sidebar/panels/site-panel/index.tsx b/packages/editor/src/components/ui/sidebar/panels/site-panel/index.tsx
index af7fe82de..07c4adf3e 100644
--- a/packages/editor/src/components/ui/sidebar/panels/site-panel/index.tsx
+++ b/packages/editor/src/components/ui/sidebar/panels/site-panel/index.tsx
@@ -50,6 +50,7 @@ import { createLocalGuideImage } from './../../../../../lib/local-guide-image'
 import { cn } from './../../../../../lib/utils'
 import useEditor from './../../../../../store/use-editor'
 import { useUploadStore } from '../../../../../store/use-upload'
+import { MetricControl } from '../../../controls/metric-control'
 import { LevelDuplicateDialog } from '../../../level-duplicate-dialog'
 import { InlineRenameInput } from './inline-rename-input'
 import { focusTreeNode, TreeNode } from './tree-node'
@@ -872,6 +873,17 @@ const LevelItem = memo(function LevelItem({
             initial={{ height: 0, opacity: 0 }}
             transition={{ type: 'spring', bounce: 0, duration: 0.3 }}
           >
+            <div className="relative border-border/50 border-b py-2 pr-3 pl-[60px]">
+              <div className="pointer-events-none absolute top-0 bottom-0 left-[45px] z-10 w-px bg-border/50" />
+              <MetricControl
+                label="Base elevation"
+                onChange={(value) => updateNode(level.id, { baseElevation: value })}
+                precision={2}
+                step={0.05}
+                unit="m"
+                value={Math.round(level.baseElevation * 100) / 100}
+              />
+            </div>
             <LevelReferences
               isLastLevel={isLast}
               levelId={level.id}
diff --git a/packages/viewer/src/systems/level/level-stacking.test.ts b/packages/viewer/src/systems/level/level-stacking.test.ts
index c71d7668a..ab6033fc1 100644
--- a/packages/viewer/src/systems/level/level-stacking.test.ts
+++ b/packages/viewer/src/systems/level/level-stacking.test.ts
@@ -64,4 +64,76 @@ describe('getLevelStackPositions', () => {
       level_1: 2.7,
     })
   })
+
+  test('applies an offset to its level and every higher level in the same building', () => {
+    const entries: LevelStackEntry[] = [
+      {
+        levelId: 'level_a0',
+        buildingId: 'building_a',
+        index: 0,
+        height: 2.5,
+        baseElevation: 0,
+      },
+      {
+        levelId: 'level_b0',
+        buildingId: 'building_b',
+        index: 0,
+        height: 3,
+        baseElevation: 0,
+      },
+      {
+        levelId: 'level_a1',
+        buildingId: 'building_a',
+        index: 1,
+        height: 3,
+        baseElevation: 1.25,
+      },
+      {
+        levelId: 'level_b1',
+        buildingId: 'building_b',
+        index: 1,
+        height: 3,
+        baseElevation: 0,
+      },
+      {
+        levelId: 'level_a2',
+        buildingId: 'building_a',
+        index: 2,
+        height: 2.8,
+        baseElevation: 0,
+      },
+    ]
+
+    expect(Object.fromEntries(getLevelStackPositions(entries))).toEqual({
+      level_a0: 0,
+      level_b0: 0,
+      level_a1: 3.75,
+      level_b1: 3,
+      level_a2: 6.75,
+    })
+  })
+
+  test('allows negative offsets', () => {
+    const entries: LevelStackEntry[] = [
+      {
+        levelId: 'level_ground',
+        buildingId: 'building_a',
+        index: 0,
+        height: 2.5,
+        baseElevation: -0.75,
+      },
+      {
+        levelId: 'level_first',
+        buildingId: 'building_a',
+        index: 1,
+        height: 3,
+        baseElevation: 0,
+      },
+    ]
+
+    expect(Object.fromEntries(getLevelStackPositions(entries))).toEqual({
+      level_ground: -0.75,
+      level_first: 1.75,
+    })
+  })
 })
diff --git a/packages/viewer/src/systems/level/level-stacking.ts b/packages/viewer/src/systems/level/level-stacking.ts
index b966e3835..af2e577ce 100644
--- a/packages/viewer/src/systems/level/level-stacking.ts
+++ b/packages/viewer/src/systems/level/level-stacking.ts
@@ -3,6 +3,7 @@ export type LevelStackEntry = {
   buildingId: string | null
   index: number
   height: number
+  baseElevation?: number
 }
 
 type BuildingOwnership = { id: string; children: readonly string[] }
@@ -24,8 +25,11 @@ export function getLevelStackPositions(entries: readonly LevelStackEntry[]): Map
 
   for (const entry of [...entries].sort((a, b) => a.index - b.index)) {
     const baseY = cumulativeYByBuilding.get(entry.buildingId) ?? 0
-    positions.set(entry.levelId, baseY)
-    cumulativeYByBuilding.set(entry.buildingId, baseY + entry.height)
+    // baseElevation is an offset, not an absolute Y: it shifts this level and,
+    // cumulatively, every level above it in the same building. Negative offsets are valid.
+    const levelY = baseY + (entry.baseElevation ?? 0)
+    positions.set(entry.levelId, levelY)
+    cumulativeYByBuilding.set(entry.buildingId, levelY + entry.height)
   }
 
   return positions
diff --git a/packages/viewer/src/systems/level/level-system.test.ts b/packages/viewer/src/systems/level/level-system.test.ts
new file mode 100644
index 000000000..f745fdf2c
--- /dev/null
+++ b/packages/viewer/src/systems/level/level-system.test.ts
@@ -0,0 +1,165 @@
+// @ts-expect-error — bun:test is provided by the Bun runtime; viewer does not
+// include Bun ambient types in its production declaration build.
+import { afterEach, describe, expect, mock, test } from 'bun:test'
+
+type FakeLevelObject = {
+  position: { y: number }
+  visible: boolean
+}
+
+type FakeLevelNode = {
+  id: string
+  type: 'level'
+  parentId: string
+  level: number
+  baseElevation: number
+  children: []
+}
+
+type FakeBuildingNode = {
+  id: string
+  type: 'building'
+  children: string[]
+}
+
+const levelIds = new Set<string>()
+const registryNodes = new Map<string, FakeLevelObject>()
+const sceneRegistry = {
+  byType: { level: levelIds },
+  nodes: registryNodes,
+}
+let nodes: Record<string, FakeLevelNode | FakeBuildingNode> = {}
+let viewerState = {
+  levelMode: 'stacked' as 'stacked' | 'exploded' | 'solo',
+  selection: { levelId: null as string | null },
+}
+let frameCallback: ((state: unknown, delta: number) => void) | null = null
+
+mock.module('@pascal-app/core', () => ({
+  getLevelHeight: (levelId: string) => (nodes[levelId]?.type === 'level' ? 2.5 : 0),
+  sceneRegistry,
+  useScene: {
+    getState: () => ({ nodes }),
+  },
+}))
+
+mock.module('@react-three/fiber', () => ({
+  useFrame: (callback: (state: unknown, delta: number) => void) => {
+    frameCallback = callback
+  },
+}))
+
+mock.module('three/src/math/MathUtils.js', () => ({
+  lerp: (start: number, end: number, alpha: number) => start + (end - start) * alpha,
+}))
+
+mock.module('../../store/use-viewer', () => ({
+  default: {
+    getState: () => viewerState,
+  },
+}))
+
+const [{ LevelSystem }, { snapLevelsToTruePositions }] = await Promise.all([
+  import('./level-system'),
+  import('./level-utils'),
+])
+
+function setupLevels(baseElevations: number[]) {
+  const buildingId = 'building_base-elevation-system-test'
+  const levels: FakeLevelNode[] = baseElevations.map((baseElevation, level) => ({
+    id: `level_base-elevation-system-${level}`,
+    type: 'level',
+    parentId: buildingId,
+    level,
+    baseElevation,
+    children: [],
+  }))
+  const building: FakeBuildingNode = {
+    id: buildingId,
+    type: 'building',
+    children: levels.map((level) => level.id),
+  }
+  nodes = Object.fromEntries([building, ...levels].map((node) => [node.id, node]))
+
+  const objects = levels.map((level) => {
+    const object: FakeLevelObject = {
+      position: { y: -100 },
+      visible: true,
+    }
+    sceneRegistry.nodes.set(level.id, object)
+    sceneRegistry.byType.level.add(level.id)
+    return object
+  })
+
+  return { building, levels, objects }
+}
+
+function setLevelMode(
+  mode: 'stacked' | 'exploded' | 'solo',
+  selectedLevelId: string | null = null,
+) {
+  viewerState = {
+    levelMode: mode,
+    selection: { levelId: selectedLevelId },
+  }
+}
+
+function updateLevelPresentation(delta: number) {
+  frameCallback = null
+  LevelSystem()
+  expect(frameCallback).not.toBeNull()
+  frameCallback?.({}, delta)
+}
+
+afterEach(() => {
+  sceneRegistry.nodes.clear()
+  sceneRegistry.byType.level.clear()
+  nodes = {}
+})
+
+describe('updateLevelPresentation', () => {
+  test('writes offset positions to the registry transform used by floorplan and selection', () => {
+    const { objects } = setupLevels([0, 1.25, 0])
+    setLevelMode('stacked')
+
+    updateLevelPresentation(1 / 12)
+
+    expect(objects.map((object) => object.position.y)).toEqual([0, 3.75, 6.25])
+  })
+
+  test('keeps offset-aware positions in exploded and solo modes', () => {
+    const { levels, objects } = setupLevels([1, 0.5])
+
+    setLevelMode('exploded')
+    updateLevelPresentation(1 / 12)
+    expect(objects.map((object) => object.position.y)).toEqual([1, 9])
+
+    objects.forEach((object) => {
+      object.position.y = -100
+    })
+    setLevelMode('solo', levels[1]!.id)
+    updateLevelPresentation(1 / 12)
+    expect(objects.map((object) => object.position.y)).toEqual([1, 4])
+    expect(objects[0]!.visible).toBe(false)
+    expect(objects[1]!.visible).toBe(true)
+  })
+})
+
+describe('snapLevelsToTruePositions', () => {
+  test('bakes offset-aware stacked positions and restores the prior presentation', () => {
+    const { objects } = setupLevels([0.5, 1.25])
+    objects[0]!.position.y = 10
+    objects[0]!.visible = false
+    objects[1]!.position.y = 20
+
+    const restore = snapLevelsToTruePositions()
+
+    expect(objects.map((object) => object.position.y)).toEqual([0.5, 4.25])
+    expect(objects.map((object) => object.visible)).toEqual([true, true])
+
+    restore()
+
+    expect(objects.map((object) => object.position.y)).toEqual([10, 20])
+    expect(objects.map((object) => object.visible)).toEqual([false, true])
+  })
+})
diff --git a/packages/viewer/src/systems/level/level-system.tsx b/packages/viewer/src/systems/level/level-system.tsx
index 80af5af36..18fd19ad0 100644
--- a/packages/viewer/src/systems/level/level-system.tsx
+++ b/packages/viewer/src/systems/level/level-system.tsx
@@ -33,6 +33,7 @@ export const LevelSystem = () => {
       buildingId: string | null
       index: number
       height: number
+      baseElevation: number
       obj: NonNullable<ReturnType<typeof sceneRegistry.nodes.get>>
     }
     const entries: LevelEntry[] = []
@@ -52,6 +53,7 @@ export const LevelSystem = () => {
             nodes,
             (wallId) => sceneRegistry.nodes.get(wallId)?.position.y,
           ),
+          baseElevation: level.baseElevation,
           obj,
         })
       }
diff --git a/packages/viewer/src/systems/level/level-utils.ts b/packages/viewer/src/systems/level/level-utils.ts
index 25c03ba94..475b2c601 100644
--- a/packages/viewer/src/systems/level/level-utils.ts
+++ b/packages/viewer/src/systems/level/level-utils.ts
@@ -28,6 +28,7 @@ export function snapLevelsToTruePositions(): () => void {
     buildingId: string | null
     index: number
     height: number
+    baseElevation: number
   }
 
   const entries: LevelEntry[] = []
@@ -47,6 +48,7 @@ export function snapLevelsToTruePositions(): () => void {
           nodes,
           (wallId) => sceneRegistry.nodes.get(wallId)?.position.y,
         ),
+        baseElevation: level.baseElevation,
         obj,
       })
     }