diff --git a/app/utils.py b/app/utils.py
index 8c1d737af..59f236101 100644
--- a/app/utils.py
+++ b/app/utils.py
@@ -23,6 +23,7 @@ from notifications_utils.formatters import (
     unescaped_formatted_list,
 )
 from notifications_utils.letter_timings import letter_can_be_cancelled
+from notifications_utils.postal_address import PostalAddress
 from notifications_utils.recipients import RecipientCSV
 from notifications_utils.take import Take
 from notifications_utils.template import (
@@ -658,6 +659,28 @@ LETTER_VALIDATION_MESSAGES = {
         'summary': (
             'Validation failed because the last line of the address is not a real UK postcode.'
         ),
+    },
+    'not-enough-address-lines': {
+        'title': 'There’s a problem with the address for this letter',
+        'detail': (
+            f'The address must be at least {PostalAddress.MIN_LINES} '
+            f'lines long.'
+        ),
+        'summary': (
+            f'Validation failed because the address must be at least '
+            f'{PostalAddress.MIN_LINES} lines long.'
+        ),
+    },
+    'too-many-address-lines': {
+        'title': 'There’s a problem with the address for this letter',
+        'detail': (
+            f'The address must be no more than {PostalAddress.MAX_LINES} '
+            f'lines long.'
+        ),
+        'summary': (
+            f'Validation failed because the address must be no more '
+            f'than {PostalAddress.MAX_LINES} lines long.'
+        ),
     }
 }
 
diff --git a/tests/app/test_utils.py b/tests/app/test_utils.py
index 58c1a3079..b91e473d6 100644
--- a/tests/app/test_utils.py
+++ b/tests/app/test_utils.py
@@ -507,6 +507,28 @@ def test_get_letter_validation_error_for_unknown_error():
             'Validation failed because the last line of the address is not a real UK postcode.'
         ),
     ),
+    (
+        'not-enough-address-lines',
+        None,
+        'There’s a problem with the address for this letter',
+        (
+            'The address must be at least 3 lines long.'
+        ),
+        (
+            'Validation failed because the address must be at least 3 lines long.'
+        ),
+    ),
+    (
+        'too-many-address-lines',
+        None,
+        'There’s a problem with the address for this letter',
+        (
+            'The address must be no more than 7 lines long.'
+        ),
+        (
+            'Validation failed because the address must be no more than 7 lines long.'
+        ),
+    ),
 ])
 def test_get_letter_validation_error_for_known_errors(
     client_request,