diff --git a/apps/viewer/src/store/slices/addElementMeshes.frame.test.ts b/apps/viewer/src/store/slices/addElementMeshes.frame.test.ts
new file mode 100644
index 000000000..303625e1b
--- /dev/null
+++ b/apps/viewer/src/store/slices/addElementMeshes.frame.test.ts
@@ -0,0 +1,273 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+/**
+ * `addElementMeshes` had no test of any kind (#2802 lists it among the slices
+ * no fixture reaches). What it does is convert IFC storey-local coordinates
+ * into the renderer's frame:
+ *
+ *   renderer.x =  ifc.x
+ *   renderer.y =  ifc.z + storeyElevation
+ *   renderer.z = -ifc.y
+ *
+ * Three of the four ways that can break — swapping the two mapped axes,
+ * dropping the negation, dropping the elevation — are INVISIBLE to a fixture
+ * built from convenient numbers. A preview at the origin, on a storey at
+ * elevation 0, with a square footprint, maps to itself under all of them.
+ *
+ * So every number below is deliberately distinct and non-zero, and the
+ * assertions are on the renderer-frame extent rather than on vertex count.
+ * A preview in the wrong place is the defect; a preview with the right number
+ * of vertices in the wrong place is the same defect.
+ */
+
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import { buildElementMesh } from './addElementMeshes.js';
+
+/** Renderer-frame bounding box of a built mesh. */
+function extent(positions: Float32Array) {
+  const lo = [Infinity, Infinity, Infinity];
+  const hi = [-Infinity, -Infinity, -Infinity];
+  for (let i = 0; i < positions.length; i += 3) {
+    for (let a = 0; a < 3; a++) {
+      lo[a] = Math.min(lo[a], positions[i + a]);
+      hi[a] = Math.max(hi[a], positions[i + a]);
+    }
+  }
+  return { lo, hi };
+}
+
+const near = (got: number, want: number, what: string) =>
+  assert.ok(Math.abs(got - want) < 1e-5, `${what}: expected ${want}, got ${got}`);
+
+describe('addElementMeshes: IFC storey-local to renderer frame (#2802)', () => {
+  it('maps a column onto the axes the renderer expects', () => {
+    // Distinct on every axis, none zero, and Width !== Depth so an X/Y size
+    // swap moves the box too.
+    const STOREY_ELEVATION = 11;
+    const mesh = buildElementMesh({
+      type: 'column',
+      globalId: 4242,
+      storeyElevation: STOREY_ELEVATION,
+      payload: {
+        type: 'column',
+        params: { Width: 1, Depth: 0.4, Height: 5 },
+        position: [3, 7, 2],
+      },
+    });
+    assert.ok(mesh, 'a well-formed column must build');
+
+    const { lo, hi } = extent(mesh.positions);
+
+    // x is carried straight through: 3 +/- Width/2.
+    near(lo[0], 2.5, 'min x');
+    near(hi[0], 3.5, 'max x');
+
+    // y is IFC z PLUS the storey elevation. Both halves matter: without the
+    // elevation this is [2, 7], which is a preview 11 m below the floor the
+    // user is standing on.
+    near(lo[1], 2 + STOREY_ELEVATION, 'min y (ifc z + elevation)');
+    near(hi[1], 2 + 5 + STOREY_ELEVATION, 'max y');
+
+    // z is NEGATED ifc y. Positive 7 becomes negative, +/- Depth/2. Drop the
+    // sign and the column appears mirrored across the model.
+    near(lo[2], -7.2, 'min z (negated ifc y)');
+    near(hi[2], -6.8, 'max z');
+
+    // A guard on the FIXTURE, not on the code: given the four assertions above
+    // pass, these spans are forced, so this cannot fail today. It exists to red
+    // a future edit that makes two dimensions equal while updating the
+    // expectations to match — which would leave the file passing and blind to
+    // an axis swap again, the failure this whole file is about.
+    const spans = [hi[0] - lo[0], hi[1] - lo[1], hi[2] - lo[2]];
+    assert.equal(
+      new Set(spans.map((s) => s.toFixed(4))).size,
+      3,
+      `the fixture must not be symmetric on any pair of axes, got spans ${spans}`
+    );
+  });
+
+  it('maps normals through the same frame change as positions', () => {
+    // A normal that is not rotated with its geometry lights the preview from
+    // the wrong side, which reads as a material bug rather than a frame bug.
+    // The bottom face is IFC [0, 0, -1], so renderer [0, -1, 0].
+    const mesh = buildElementMesh({
+      type: 'column',
+      globalId: 1,
+      storeyElevation: 0,
+      payload: { type: 'column', params: { Width: 1, Depth: 2, Height: 3 }, position: [0, 0, 0] },
+    });
+    assert.ok(mesh);
+    const n = mesh.normals;
+    near(n[0], 0, 'bottom normal x');
+    near(n[1], -1, 'bottom normal y (IFC -Z becomes renderer -Y)');
+    near(n[2], 0, 'bottom normal z');
+
+    // The bottom face is [0, 0, -1] in IFC, so its x and y are BOTH zero and
+    // it cannot see the `-face.normal[1]` term at all: flip that sign and the
+    // assertions above still pass. Vertices 8..11 are the first side face,
+    // IFC normal [0, 1, 0], which maps to renderer [0, 0, -1] and is the
+    // component that discriminates.
+    near(n[24], 0, 'side normal x');
+    near(n[25], 0, 'side normal y');
+    near(n[26], -1, 'side normal z (IFC +Y becomes renderer -Z)');
+
+    // Every normal is unit length, so none was left un-normalised by the swap.
+    // Note this alone cannot see a sign flip, which is why the component
+    // assertions above exist.
+    for (let i = 0; i < n.length; i += 3) {
+      const len = Math.hypot(n[i], n[i + 1], n[i + 2]);
+      assert.ok(Math.abs(len - 1) < 1e-4, `normal ${i / 3} has length ${len}`);
+    }
+  });
+
+  it('maps the polygon path too, which transcribes the frame change separately', () => {
+    // `buildPolygonExtrusion` does not share `buildBoxFromIfcCorners`. It
+    // carries its OWN copy of `(x, z + elevation, -y)`, so the three tests
+    // above pin none of it: dropping the elevation or the negation there fails
+    // nothing they assert.
+    //
+    // That matters beyond symmetry with the box path. Of the two production
+    // callers, `useSpaceGhostPreview` builds ONLY `type: 'space'` payloads, so
+    // one entire caller exercises exclusively the copy the box tests miss.
+    //
+    // Two transcriptions of one rule with no gate is the shape this repo keeps
+    // getting caught by. Pinning both is the cheap half; collapsing them to one
+    // is the real fix and belongs in a change that touches production code.
+    const STOREY_ELEVATION = 4;
+    const HEIGHT = 0.3;
+    // `space`, not `slab`, because the caller this test cites builds only
+    // `space` payloads — and the two take DIFFERENT lines in the dispatch
+    // (`params.Height` versus `params.Thickness`), so covering slab would have
+    // left the cited path unpinned while claiming to cover it.
+    //
+    // A triangle, asymmetric on both axes, clear of the origin, and with a
+    // DISTINCT z per corner: the extrusion takes its base from corner[0] and
+    // flattens the rest onto that plane, which a footprint at one z cannot
+    // tell apart from per-corner z or from any other corner's.
+    const mesh = buildElementMesh({
+      type: 'space',
+      globalId: 77,
+      storeyElevation: STOREY_ELEVATION,
+      payload: {
+        type: 'space',
+        params: { Width: 6, Depth: 4, Height: HEIGHT },
+        corners: [
+          [2, 5, 1],
+          [8, 5, 3],
+          [2, 9, 7],
+        ],
+      },
+    });
+    assert.ok(mesh, 'a well-formed space must build');
+
+    const { lo, hi } = extent(mesh.positions);
+    near(lo[0], 2, 'min x');
+    near(hi[0], 8, 'max x');
+    // baseZ is corner[0]'s z (1) and ONLY corner[0]'s. With corners at z 1, 3
+    // and 7, reading the last corner or reading per-corner z both put max y at
+    // 11.3 instead of 5.3.
+    near(lo[1], 1 + STOREY_ELEVATION, 'min y (corner[0] ifc z + elevation)');
+    near(hi[1], 1 + HEIGHT + STOREY_ELEVATION, 'max y');
+    // Negated, so ifc y of 5..9 becomes renderer z of -9..-5.
+    near(lo[2], -9, 'min z (negated ifc y)');
+    near(hi[2], -5, 'max z');
+  });
+
+  it('rejects a zero-height space rather than emitting a flat volume', () => {
+    // Valid-but-falsy: `Height: 0` from the panel is a number, passes any
+    // truthiness check, and produces a zero-volume preview. The guard exists;
+    // nothing reached it.
+    const flat = buildElementMesh({
+      type: 'space',
+      globalId: 5,
+      storeyElevation: 0,
+      payload: {
+        type: 'space',
+        params: { Width: 1, Depth: 1, Height: 0 },
+        corners: [[0, 0, 0], [1, 0, 0], [0, 1, 0]],
+      },
+    });
+    assert.equal(flat, null, 'a zero-height space must not preview');
+  });
+
+  it('builds a sloped beam across its axis, honouring both endpoint heights', () => {
+    // `buildLinearBox` had no positive coverage at all. Its frame mapping is
+    // the shared, tested one, but everything it does BEFORE that was unpinned:
+    // the perpendicular offset, both endpoints' Z, and which param becomes the
+    // thickness. All three survive mutation without this.
+    const mesh = buildElementMesh({
+      type: 'beam',
+      globalId: 3,
+      storeyElevation: 0,
+      payload: {
+        type: 'beam',
+        params: { Width: 0.3, Height: 1 },
+        start: [1, 2, 0],
+        end: [5, 2, 2], // sloped: end is 2 m higher than start
+      },
+    });
+    assert.ok(mesh, 'a well-formed beam must build');
+    const { lo, hi } = extent(mesh.positions);
+
+    // Along the axis (renderer x): the segment's own extent, unpadded.
+    near(lo[0], 1, 'min x follows the start point');
+    near(hi[0], 5, 'max x follows the end point');
+
+    // Across it: Width/2 either side of ifc y = 2, negated. Extruding along
+    // the axis instead of across collapses this to a zero span.
+    near(lo[2], -2.15, 'min z (thickness across the axis, negated)');
+    near(hi[2], -1.85, 'max z');
+
+    // Both endpoint Zs: start 0, end 2, plus Height 1 on top of the higher
+    // end. Pinning the base to `startIfc[2]` — which the source comment says
+    // it deliberately does not do — caps this at 1.
+    near(lo[1], 0, 'min y follows the LOWER endpoint');
+    near(hi[1], 3, 'max y follows the HIGHER endpoint plus Height');
+  });
+
+  it('gives a door its frame thickness across, not as its height', () => {
+    // door and window call `buildAxisBox(..., Width, FrameThickness, Height)`
+    // where column passes `(Width, Depth, Height)`. That per-type argument
+    // order is a SECOND transcription, separate from the frame mapping, and
+    // swapping the last two gives every door a 0.05 m tall, 2 m deep preview.
+    const mesh = buildElementMesh({
+      type: 'door',
+      globalId: 8,
+      storeyElevation: 0,
+      payload: {
+        type: 'door',
+        params: { Width: 0.9, Height: 2.1, FrameThickness: 0.05 },
+        position: [0, 0, 0],
+      },
+    });
+    assert.ok(mesh);
+    const { lo, hi } = extent(mesh.positions);
+    near(hi[0] - lo[0], 0.9, 'width spans X');
+    near(hi[1] - lo[1], 2.1, 'HEIGHT spans renderer Y, not the frame thickness');
+    near(hi[2] - lo[2], 0.05, 'frame thickness spans renderer Z');
+  });
+
+  it('refuses a degenerate wall instead of emitting a zero-size preview', () => {
+    // Control on the tests above, which assert a mesh IS built and would also
+    // hold if this function built one for anything at all.
+    //
+    // The mirror hazard is covered by the beam test above rather than here: a
+    // guard that rejects VALID input leaves this assertion green too, and in
+    // this repo that is the more common of the two.
+    const degenerate = buildElementMesh({
+      type: 'wall',
+      globalId: 9,
+      storeyElevation: 0,
+      payload: {
+        type: 'wall',
+        params: { Thickness: 0.2, Height: 3 },
+        start: [1, 1, 0],
+        end: [1, 1, 0], // zero length
+      },
+    });
+    assert.equal(degenerate, null, 'a zero-length wall must not produce a preview');
+  });
+});