diff --git a/.github/workflows/deploy_terraform.yml b/.github/workflows/deploy_terraform.yml
index 5e469dc66..8f2d405f4 100644
--- a/.github/workflows/deploy_terraform.yml
+++ b/.github/workflows/deploy_terraform.yml
@@ -534,11 +534,37 @@ jobs:
       TF_VAR_social_housing_wave_3_sharepoint_id: ${{ secrets.SOCIAL_HOUSING_WAVE_3_SHAREPOINT_ID }}
 
 
+  # ============================================================
+  # Check FastAPI Lambda package will fit AWS's unzipped size limit
+  # ============================================================
+  fast_api_lambda_zip_size_check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+
+      - name: Check projected unzipped Lambda size
+        run: |
+          python3 scripts/check_lambda_zip_size.py \
+            --source-dir . \
+            --requirements backend/app/requirements/requirements.txt \
+            --exclude "**/__pycache__/**" \
+            --exclude "**/*.pyc" \
+            --exclude "**/.pytest_cache/**" \
+            --exclude "**/tests/**" \
+            --exclude "**/deployment/**" \
+            --exclude "**/.git/**" \
+            --exclude "**/json_samples/**" \
+            --exclude "**/docs/specs/**"
+
   # ============================================================
   # Deploy FastAPI Lambda
   # ============================================================
   fast_api_lambda:
-    needs: [determine_stage, ara_engine_lambda, categorisation_lambda, postcodeSplitter_lambda, bulk_address2uprn_combiner_lambda, bulkUploadFinaliser_lambda]
+    needs: [determine_stage, ara_engine_lambda, categorisation_lambda, postcodeSplitter_lambda, bulk_address2uprn_combiner_lambda, bulkUploadFinaliser_lambda, fast_api_lambda_zip_size_check]
     uses: ./.github/workflows/_deploy_lambda.yml
     with:
       lambda_name: ara_fast_api
diff --git a/deployment/terraform/modules/lambda_with_api_gateway/variables.tf b/deployment/terraform/modules/lambda_with_api_gateway/variables.tf
index b5d0515a0..7576a4bf0 100644
--- a/deployment/terraform/modules/lambda_with_api_gateway/variables.tf
+++ b/deployment/terraform/modules/lambda_with_api_gateway/variables.tf
@@ -11,7 +11,10 @@ variable "zip_excludes" {
     "**/*.pyc",
     "**/.pytest_cache/**",
     "**/tests/**",
-    "**/deployment/**"
+    "**/deployment/**",
+    "**/.git/**",
+    "**/json_samples/**",
+    "**/docs/specs/**"
   ]
 }
 
diff --git a/scripts/check_lambda_zip_size.py b/scripts/check_lambda_zip_size.py
new file mode 100755
index 000000000..160ac4b63
--- /dev/null
+++ b/scripts/check_lambda_zip_size.py
@@ -0,0 +1,149 @@
+#!/usr/bin/env python3
+"""Estimate the unzipped size of a Terraform-packaged Lambda before deploying.
+
+Mirrors what `deployment/terraform/modules/lambda_with_api_gateway/main.tf` does:
+`pip install -r <requirements> -t <source_dir> ...` followed by zipping
+`source_dir` minus `zip_excludes`. AWS Lambda's hard limit is 262144000 bytes
+(250 MiB) unzipped. We install the dependencies into a scratch directory
+instead of the real source tree so this can run locally or in CI without
+mutating the repo.
+
+Usage:
+    python3 scripts/check_lambda_zip_size.py \
+        --source-dir . \
+        --requirements backend/app/requirements/requirements.txt \
+        --exclude "**/tests/**" --exclude "**/.git/**" ...
+
+Exits non-zero if the projected unzipped size exceeds --limit (or exceeds
+--warn-pct of the limit, when --strict is not passed the warning is just
+printed).
+"""
+from __future__ import annotations
+
+import argparse
+import fnmatch
+import subprocess
+import sys
+import tempfile
+from pathlib import Path
+
+LAMBDA_UNZIPPED_LIMIT_BYTES = 262_144_000
+
+
+def doublestar_match(pattern_parts: list[str], path_parts: list[str]) -> bool:
+    if not pattern_parts:
+        return not path_parts
+    head, rest = pattern_parts[0], pattern_parts[1:]
+    if head == "**":
+        if doublestar_match(rest, path_parts):
+            return True
+        if path_parts and doublestar_match(pattern_parts, path_parts[1:]):
+            return True
+        return False
+    if not path_parts:
+        return False
+    if fnmatch.fnmatchcase(path_parts[0], head):
+        return doublestar_match(rest, path_parts[1:])
+    return False
+
+
+def is_excluded(rel_path: Path, excludes: list[str]) -> bool:
+    parts = rel_path.as_posix().split("/")
+    return any(doublestar_match(pattern.split("/"), parts) for pattern in excludes)
+
+
+def sum_dir_size(root: Path, excludes: list[str] | None = None) -> int:
+    total = 0
+    for path in root.rglob("*"):
+        if path.is_dir():
+            continue
+        rel = path.relative_to(root)
+        if excludes and is_excluded(rel, excludes):
+            continue
+        total += path.stat().st_size
+    return total
+
+
+def install_dependencies(requirements: Path, target: Path) -> int:
+    subprocess.run(
+        [
+            "pip",
+            "install",
+            "-r",
+            str(requirements),
+            "-t",
+            str(target),
+            "--platform",
+            "manylinux2014_x86_64",
+            "--implementation",
+            "cp",
+            "--python-version",
+            "3.11",
+            "--only-binary=:all:",
+        ],
+        check=True,
+    )
+    return sum_dir_size(target)
+
+
+def human(n: int) -> str:
+    return f"{n:,} bytes ({n / 1024 / 1024:.1f} MiB)"
+
+
+def main() -> int:
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--source-dir", type=Path, default=Path("."))
+    parser.add_argument("--requirements", type=Path, default=None)
+    parser.add_argument("--exclude", action="append", default=[], dest="excludes")
+    parser.add_argument("--limit", type=int, default=LAMBDA_UNZIPPED_LIMIT_BYTES)
+    parser.add_argument(
+        "--warn-pct",
+        type=float,
+        default=85.0,
+        help="Print a warning once projected size crosses this %% of the limit.",
+    )
+    parser.add_argument(
+        "--strict",
+        action="store_true",
+        help="Exit non-zero on the warning threshold too, not just the hard limit.",
+    )
+    args = parser.parse_args()
+
+    source_size = sum_dir_size(args.source_dir, args.excludes)
+
+    deps_size = 0
+    if args.requirements is not None:
+        with tempfile.TemporaryDirectory(prefix="lambda-deps-") as tmp:
+            deps_size = install_dependencies(args.requirements, Path(tmp))
+
+    total = source_size + deps_size
+    pct = total / args.limit * 100
+
+    print(f"source files (post-exclude): {human(source_size)}")
+    if args.requirements is not None:
+        print(f"pip dependencies:            {human(deps_size)}")
+    print(f"projected unzipped total:     {human(total)}  [{pct:.1f}% of limit]")
+    print(f"AWS limit:                    {human(args.limit)}")
+
+    if total > args.limit:
+        print(
+            f"\nFAIL: projected unzipped size exceeds the Lambda limit by "
+            f"{human(total - args.limit)}.",
+            file=sys.stderr,
+        )
+        return 1
+
+    if pct >= args.warn_pct:
+        msg = (
+            f"\nWARNING: projected unzipped size is at {pct:.1f}% of the "
+            f"{human(args.limit)} limit."
+        )
+        print(msg, file=sys.stderr)
+        if args.strict:
+            return 1
+
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())