diff --git a/README.md b/README.md
index f2cf36e..313f109 100644
--- a/README.md
+++ b/README.md
@@ -71,6 +71,8 @@ function name unique per region, for example by setting
 | attach\_dead\_letter\_config | Set this to true if using the dead_letter_config variable | string | `"false"` | no |
 | attach\_policy | Set this to true if using the policy variable | string | `"false"` | no |
 | attach\_vpc\_config | Set this to true if using the vpc_config variable | string | `"false"` | no |
+| build\_command | The command that creates the Lambda package zip file | string | `"python build.py '$filename' '$runtime' '$source'"` | no |
+| build\_paths | The files or directories used by the build command, to trigger new Lambda package builds whenever build scripts change | list | `<list>` | no |
 | dead\_letter\_config | Dead letter configuration for the Lambda function | map | `<map>` | no |
 | description | Description of what your Lambda function does | string | `"Managed by Terraform"` | no |
 | enable\_cloudwatch\_logs | Set this to false to disable logging your Lambda output to CloudWatch Logs | string | `"true"` | no |
diff --git a/archive.tf b/archive.tf
index 9f5be61..76596b3 100644
--- a/archive.tf
+++ b/archive.tf
@@ -1,11 +1,18 @@
+locals {
+  module_relpath = "${substr(path.module, length(path.cwd) + 1, -1)}"
+}
+
 # Generates a filename for the zip archive based on the contents of the files
 # in source_path. The filename will change when the source code changes.
 data "external" "archive" {
   program = ["python", "${path.module}/hash.py"]
 
   query = {
-    runtime     = "${var.runtime}"
-    source_path = "${var.source_path}"
+    build_command  = "${var.build_command}"
+    build_paths    = "${jsonencode(var.build_paths)}"
+    module_relpath = "${local.module_relpath}"
+    runtime        = "${var.runtime}"
+    source_path    = "${var.source_path}"
   }
 }
 
@@ -16,7 +23,8 @@ resource "null_resource" "archive" {
   }
 
   provisioner "local-exec" {
-    command = "${lookup(data.external.archive.result, "build_command")}"
+    command     = "${lookup(data.external.archive.result, "build_command")}"
+    working_dir = "${path.module}"
   }
 }
 
@@ -29,8 +37,9 @@ data "external" "built" {
   program = ["python", "${path.module}/built.py"]
 
   query = {
-    build_command = "${lookup(data.external.archive.result, "build_command")}"
-    filename_old  = "${lookup(null_resource.archive.triggers, "filename")}"
-    filename_new  = "${lookup(data.external.archive.result, "filename")}"
+    build_command  = "${lookup(data.external.archive.result, "build_command")}"
+    filename_old   = "${lookup(null_resource.archive.triggers, "filename")}"
+    filename_new   = "${lookup(data.external.archive.result, "filename")}"
+    module_relpath = "${local.module_relpath}"
   }
 }
diff --git a/build.py b/build.py
index faf8730..33304d8 100644
--- a/build.py
+++ b/build.py
@@ -1,8 +1,6 @@
 # Builds a zip file from the source_dir or source_file.
 # Installs dependencies with pip automatically.
 
-import base64
-import json
 import os
 import shutil
 import subprocess
@@ -105,11 +103,10 @@ def create_zip_file(source_dir, target_file):
         root_dir=source_dir,
     )
 
-json_payload = bytes.decode(base64.b64decode(sys.argv[1]))
-query = json.loads(json_payload)
-filename = query['filename']
-runtime = query['runtime']
-source_path = query['source_path']
+
+filename = sys.argv[1]
+runtime = sys.argv[2]
+source_path = sys.argv[3]
 
 absolute_filename = os.path.abspath(filename)
 
diff --git a/builds/.gitignore b/builds/.gitignore
new file mode 100644
index 0000000..c4c4ffc
--- /dev/null
+++ b/builds/.gitignore
@@ -0,0 +1 @@
+*.zip
diff --git a/built.py b/built.py
index 6f96061..3902f9f 100644
--- a/built.py
+++ b/built.py
@@ -12,6 +12,7 @@
 build_command = query['build_command']
 filename_old = query['filename_old']
 filename_new = query['filename_new']
+module_relpath = query['module_relpath']
 
 # If the old filename (from the Terraform state) matches the new filename
 # (from hash.py) then the source code has not changed and thus the zip file
@@ -29,10 +30,10 @@
         # console) then it is possible that Terraform will try to upload
         # the missing file. I don't know how to tell if Terraform is going
         # to try to upload the file or not, so always ensure the file exists.
-        subprocess.check_output(build_command, shell=True)
+        subprocess.check_output(build_command, shell=True, cwd=module_relpath)
 
 # Output the filename to Terraform.
 json.dump({
-    'filename': filename_new,
+    'filename': module_relpath + '/' + filename_new,
 }, sys.stdout, indent=2)
 sys.stdout.write('\n')
diff --git a/hash.py b/hash.py
index 1f9bf44..7fc73ef 100644
--- a/hash.py
+++ b/hash.py
@@ -3,20 +3,14 @@
 #
 # Outputs a filename and a command to run if the archive needs to be built.
 
-import base64
 import datetime
 import errno
 import hashlib
 import json
 import os
-import re
 import sys
 
 
-FILENAME_PREFIX = 'terraform-aws-lambda-'
-FILENAME_PATTERN = re.compile(r'^' + FILENAME_PREFIX + r'[0-9a-f]{64}\.zip$')
-
-
 def abort(message):
     """
     Exits with an error message.
@@ -36,24 +30,21 @@ def delete_old_archives():
     now = datetime.datetime.now()
     delete_older_than = now - datetime.timedelta(days=7)
 
-    top = '.terraform'
-    if os.path.isdir(top):
-        for name in os.listdir(top):
-            if FILENAME_PATTERN.match(name):
-                path = os.path.join(top, name)
-                try:
-                    file_modified = datetime.datetime.fromtimestamp(
-                        os.path.getmtime(path)
-                    )
-                    if file_modified < delete_older_than:
-                        os.remove(path)
-                except OSError as error:
-                    if error.errno == errno.ENOENT:
-                        # Ignore "not found" errors as they are probably race
-                        # conditions between multiple usages of this module.
-                        pass
-                    else:
-                        raise
+    for name in os.listdir('builds'):
+        if name.endswith('.zip'):
+            try:
+                file_modified = datetime.datetime.fromtimestamp(
+                    os.path.getmtime(name)
+                )
+                if file_modified < delete_older_than:
+                    os.remove(name)
+            except OSError as error:
+                if error.errno == errno.ENOENT:
+                    # Ignore "not found" errors as they are probably race
+                    # conditions between multiple usages of this module.
+                    pass
+                else:
+                    raise
 
 
 def list_files(top_path):
@@ -72,22 +63,23 @@ def list_files(top_path):
     return results
 
 
-def generate_content_hash(source_path):
+def generate_content_hash(source_paths):
     """
-    Generate a content hash of the source path.
+    Generate a content hash of the source paths.
 
     """
 
     sha256 = hashlib.sha256()
 
-    if os.path.isdir(source_path):
-        source_dir = source_path
-        for source_file in list_files(source_dir):
+    for source_path in source_paths:
+        if os.path.isdir(source_path):
+            source_dir = source_path
+            for source_file in list_files(source_dir):
+                update_hash(sha256, source_dir, source_file)
+        else:
+            source_dir = os.path.dirname(source_path)
+            source_file = source_path
             update_hash(sha256, source_dir, source_file)
-    else:
-        source_dir = os.path.dirname(source_path)
-        source_file = source_path
-        update_hash(sha256, source_dir, source_file)
 
     return sha256
 
@@ -109,17 +101,11 @@ def update_hash(hash_obj, file_root, file_path):
             hash_obj.update(data)
 
 
-
-current_dir = os.path.dirname(__file__)
-
 # Parse the query.
-if len(sys.argv) > 1 and sys.argv[1] == '--test':
-    query = {
-        'runtime': 'python3.6',
-        'source_path': os.path.join(current_dir, 'tests', 'python3-pip', 'lambda'),
-    }
-else:
-    query = json.load(sys.stdin)
+query = json.load(sys.stdin)
+build_command = query['build_command']
+build_paths = json.loads(query['build_paths'])
+module_relpath = query['module_relpath']
 runtime = query['runtime']
 source_path = query['source_path']
 
@@ -127,33 +113,30 @@ def update_hash(hash_obj, file_root, file_path):
 if not source_path:
     abort('source_path must be set.')
 
+# Change working directory to the module path
+# so references to build.py will work.
+os.chdir(module_relpath)
+
 # Generate a hash based on file names and content. Also use the
-# runtime value and content of build.py because they can have an
-# effect on the resulting archive.
-content_hash = generate_content_hash(source_path)
+# runtime value, build command, and content of the build paths
+# because they can have an effect on the resulting archive.
+content_hash = generate_content_hash([source_path] + build_paths)
 content_hash.update(runtime.encode())
-with open(os.path.join(current_dir, 'build.py'), 'rb') as build_script_file:
-    content_hash.update(build_script_file.read())
+content_hash.update(build_command.encode())
 
 # Generate a unique filename based on the hash.
-filename = '.terraform/{prefix}{content_hash}.zip'.format(
-    prefix=FILENAME_PREFIX,
+filename = 'builds/{content_hash}.zip'.format(
     content_hash=content_hash.hexdigest(),
 )
 
-# Determine the command to run if Terraform wants to build a new archive.
-build_command = "python {build_script} {build_data}".format(
-    build_script=os.path.join(current_dir, 'build.py'),
-    build_data=bytes.decode(base64.b64encode(str.encode(
-        json.dumps({
-            'filename': filename,
-            'source_path': source_path,
-            'runtime': runtime,
-            })
-         )
-      ),
-   )
-)
+# Replace variables in the build command with calculated values.
+replacements = {
+    '$filename': filename,
+    '$runtime': runtime,
+    '$source': source_path,
+}
+for old, new in replacements.items():
+    build_command = build_command.replace(old, new)
 
 # Delete previous archives.
 delete_old_archives()
diff --git a/tests/.tool-versions b/tests/.tool-versions
new file mode 100644
index 0000000..a0c26f2
--- /dev/null
+++ b/tests/.tool-versions
@@ -0,0 +1 @@
+terraform 0.11.11
diff --git a/tests/build-command/lambda/build.sh b/tests/build-command/lambda/build.sh
new file mode 100755
index 0000000..b9e90b0
--- /dev/null
+++ b/tests/build-command/lambda/build.sh
@@ -0,0 +1,39 @@
+#!/bin/bash
+# 
+# Compiles a Python package into a zip deployable on AWS Lambda.
+#
+# - Builds Python dependencies into the package, using a Docker image to
+#   correctly build native extensions
+# - Able to be used with the terraform-aws-lambda module
+#
+# Dependencies:
+#
+# - Docker
+#
+# Usage:
+#
+# $ ./build.sh <output-zip-filename> <runtime> <source-path>
+
+set -euo pipefail
+
+# Read variables from command line arguments
+FILENAME=$1
+RUNTIME=$2
+SOURCE_PATH=$3
+
+# Convert to absolute paths
+SOURCE_DIR=$(cd "$SOURCE_PATH" && pwd)
+ZIP_DIR=$(cd "$(dirname "$FILENAME")" && pwd)
+ZIP_NAME=$(basename "$FILENAME")
+
+# Install dependencies, using a Docker image to correctly build native extensions
+docker run --rm -t -v "$SOURCE_DIR:/src" -v "$ZIP_DIR:/out" lambci/lambda:build-$RUNTIME sh -c "
+    cp -r /src /build &&
+    cd /build &&
+    pip install --progress-bar off -r requirements.txt -t . &&
+    chmod -R 755 . &&
+    zip -r /out/$ZIP_NAME * &&
+    chown \$(stat -c '%u:%g' /out) /out/$ZIP_NAME
+"
+
+echo "Created $FILENAME from $SOURCE_PATH"
diff --git a/tests/build-command/lambda/src/main.py b/tests/build-command/lambda/src/main.py
new file mode 100644
index 0000000..de9967e
--- /dev/null
+++ b/tests/build-command/lambda/src/main.py
@@ -0,0 +1,6 @@
+def lambda_handler(event, context):
+    print('importing numpy package')
+    import numpy as np  
+    print('checking numpy works correctly')
+    assert np.array_equal(np.array([1, 2]) + 3, np.array([4, 5]))
+    return 'test passed'
diff --git a/tests/build-command/lambda/src/requirements.txt b/tests/build-command/lambda/src/requirements.txt
new file mode 100644
index 0000000..d9c3d91
--- /dev/null
+++ b/tests/build-command/lambda/src/requirements.txt
@@ -0,0 +1,3 @@
+# numpy has native extensions, needs a custom build script to 
+# install correctly if your host OS differs to Lambda OS
+numpy
diff --git a/tests/build-command/main.tf b/tests/build-command/main.tf
new file mode 100644
index 0000000..382e067
--- /dev/null
+++ b/tests/build-command/main.tf
@@ -0,0 +1,23 @@
+terraform {
+  backend "local" {
+    path = "terraform.tfstate"
+  }
+}
+
+provider "aws" {
+  region = "eu-west-1"
+}
+
+module "lambda" {
+  source = "../../"
+
+  function_name = "terraform-aws-lambda-test-build-command"
+  description   = "Test custom build command functionality in terraform-aws-lambda"
+  handler       = "main.lambda_handler"
+  runtime       = "python3.7"
+
+  source_path = "${path.module}/lambda/src"
+
+  build_command = "${path.module}/lambda/build.sh '$filename' '$runtime' '$source'"
+  build_paths   = ["${path.module}/lambda/build.sh"]
+}
diff --git a/variables.tf b/variables.tf
index 8124e5a..2a64535 100644
--- a/variables.tf
+++ b/variables.tf
@@ -36,6 +36,18 @@ variable "source_path" {
   type        = "string"
 }
 
+variable "build_command" {
+  description = "The command that creates the Lambda package zip file"
+  type        = "string"
+  default     = "python build.py '$filename' '$runtime' '$source'"
+}
+
+variable "build_paths" {
+  description = "The files or directories used by the build command, to trigger new Lambda package builds whenever build scripts change"
+  type        = "list"
+  default     = ["build.py"]
+}
+
 variable "description" {
   description = "Description of what your Lambda function does"
   type        = "string"