diff --git a/data/processing/subdomain.py b/data/processing/subdomain.py
index 6ddcf40..2983bf1 100644
--- a/data/processing/subdomain.py
+++ b/data/processing/subdomain.py
@@ -2,6 +2,7 @@ import argparse
 import configparser
 import logging
 import pathlib
+import sys
 
 import numpy as np
 
@@ -36,6 +37,9 @@ out_root = pathlib.Path(config.get("out", "root"))
 out_root.mkdir(exist_ok=True)
 bathy_inp = inp_root.joinpath(config.get("inp", "base"))
 bathy_out = out_root.joinpath(config.get("out", "sub"))
+if bathy_out.exists():
+    log.error(f"'{bathy_out}' already exists!")
+    sys.exit(1)
 log.info(f"Reading bathymetry from '{bathy_inp}'")
 raw_bathy = np.loadtxt(bathy_inp)
 log.debug(f"Initial size: {raw_bathy.shape}")
@@ -48,3 +52,4 @@ log.debug(f"Final size: {bathy.shape}")
 
 log.info(f"Saving subdomain to 'bathy'")
 np.save(bathy_out, bathy)
+bathy_out.chmod(0o444)