diff --git a/src/core/copy.ts b/src/core/copy.ts
new file mode 100644
index 0000000..29a1dd4
--- /dev/null
+++ b/src/core/copy.ts
@@ -0,0 +1,62 @@
+import type { EditableKey } from './changes';
+import type { PierInput } from './types';
+
+/**
+ * 속성 복사 (spec §9 · plan C7). Copies chosen attributes from one pier to others.
+ * Values the spec never copies (coordinates, IP, elevations, names) are not in any item, so they cannot be copied.
+ */
+export type CopyItem = 'coping-width' | 'coping-length' | 'pile-length';
+
+interface ItemRule {
+  label: string;
+  keys: EditableKey[];
+  /** Off by default (spec: 말뚝 길이·코핑 길이 기본 해제). */
+  defaultOn: boolean;
+  /** Pile items only apply when the foundation type matches (spec §9 건너뜀). */
+  pileItem: boolean;
+}
+
+export const COPY_ITEMS: Record<CopyItem, ItemRule> = {
+  'coping-width': { label: '코핑 폭', keys: ['copingWidthMm'], defaultOn: true, pileItem: false },
+  'coping-length': { label: '코핑 길이 L', keys: ['copingLengthMm'], defaultOn: false, pileItem: false },
+  'pile-length': { label: '말뚝 길이', keys: ['pileLengthMm'], defaultOn: false, pileItem: true },
+};
+
+export const DEFAULT_COPY_ITEMS: CopyItem[] = (Object.keys(COPY_ITEMS) as CopyItem[]).filter((k) => COPY_ITEMS[k].defaultOn);
+
+export interface CopyChange {
+  key: EditableKey;
+  from: number;
+  to: number;
+}
+
+export interface CopyPlan {
+  changes: CopyChange[];
+  /** Items not applied to this target, with the reason shown in the preview. */
+  skipped: { item: CopyItem; reason: string }[];
+}
+
+/** Preview for one target: what would change and what is skipped. Same values are not counted as changes. */
+export function planCopy(source: PierInput, target: PierInput, items: CopyItem[]): CopyPlan {
+  const changes: CopyChange[] = [];
+  const skipped: CopyPlan['skipped'] = [];
+  for (const item of items) {
+    const rule = COPY_ITEMS[item];
+    if (rule.pileItem && source.foundationType !== target.foundationType) {
+      skipped.push({ item, reason: `기초 형식이 다릅니다 (${target.foundationType})` });
+      continue;
+    }
+    for (const key of rule.keys) {
+      if (source[key] === target[key]) continue;
+      changes.push({ key, from: target[key], to: source[key] });
+    }
+  }
+  return { changes, skipped };
+}
+
+/** Applies a plan to a target input. Only the planned keys change. */
+export function applyCopy(target: PierInput, plan: CopyPlan): PierInput {
+  const next = { ...target };
+  for (const c of plan.changes) next[c.key] = c.to;
+  return next;
+}
diff --git a/tests/copy.test.ts b/tests/copy.test.ts
new file mode 100644
index 0000000..e0afd6e
--- /dev/null
+++ b/tests/copy.test.ts
@@ -0,0 +1,49 @@
+import { describe, expect, it } from 'vitest';
+import { applyCopy, DEFAULT_COPY_ITEMS, planCopy } from '../src/core/copy';
+import type { PierInput } from '../src/core/types';
+import { loadP1 } from './helpers';
+
+const source = (): PierInput => ({ ...loadP1(), copingWidthMm: 3300, pileLengthMm: 20000, copingLengthMm: 11000 });
+const target = (): PierInput => ({ ...loadP1(), name: 'P2', copingWidthMm: 3100, pileLengthMm: 24000, copingLengthMm: 12000 });
+
+describe('속성 복사 engine (spec §9, plan C7)', () => {
+  it('defaults copy only the coping width; length items are off', () => {
+    expect(DEFAULT_COPY_ITEMS).toEqual(['coping-width']);
+    const plan = planCopy(source(), target(), DEFAULT_COPY_ITEMS);
+    expect(plan.changes).toEqual([{ key: 'copingWidthMm', from: 3100, to: 3300 }]);
+  });
+
+  it('identity, coordinates, IPs, bearings and elevations are never copied', () => {
+    const src = { ...source(), name: 'X', ips: [], bearings: [], copingTopEl: 99, groundEl: 99, footingBottomEl: 99 };
+    const tgt = target();
+    const next = applyCopy(tgt, planCopy(src, tgt, ['coping-width', 'coping-length', 'pile-length']));
+    expect(next.name).toBe('P2');
+    expect(next.ips).toEqual(tgt.ips);
+    expect(next.bearings).toEqual(tgt.bearings);
+    expect(next.copingTopEl).toBe(tgt.copingTopEl);
+    expect(next.groundEl).toBe(tgt.groundEl);
+    expect(next.footingBottomEl).toBe(tgt.footingBottomEl);
+  });
+
+  it('a pile item is skipped when the foundation type differs', () => {
+    const tgt = { ...target(), foundationType: '직접기초' as const };
+    const plan = planCopy(source(), tgt, ['pile-length']);
+    expect(plan.changes).toEqual([]);
+    expect(plan.skipped).toEqual([{ item: 'pile-length', reason: '기초 형식이 다릅니다 (직접기초)' }]);
+  });
+
+  it('equal values are not counted as changes', () => {
+    const src = source();
+    const tgt = { ...target(), copingWidthMm: 3300 };
+    expect(planCopy(src, tgt, ['coping-width']).changes).toEqual([]);
+  });
+
+  it('the preview count equals the number of applied changes', () => {
+    const plan = planCopy(source(), target(), ['coping-width', 'coping-length', 'pile-length']);
+    const next = applyCopy(target(), plan);
+    expect(plan.changes).toHaveLength(3);
+    expect(next.copingWidthMm).toBe(3300);
+    expect(next.copingLengthMm).toBe(11000);
+    expect(next.pileLengthMm).toBe(20000);
+  });
+});