diff --git a/subiquity/server/controllers/filesystem.py b/subiquity/server/controllers/filesystem.py
index c9d77b3d..24df42f2 100644
--- a/subiquity/server/controllers/filesystem.py
+++ b/subiquity/server/controllers/filesystem.py
@@ -133,19 +133,20 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
                 "autoinstall config did not create needed bootloader "
                 "partition")
 
-    def setup_disk_for_guided(self, disk):
-        self.reformat(disk, wipe='superblock-recursive')
+    def setup_disk_for_guided(self, disk, mode):
+        if mode is None or mode == 'reformat_disk':
+            self.reformat(disk, wipe='superblock-recursive')
         if DeviceAction.TOGGLE_BOOT in DeviceAction.supported(disk):
             self.add_boot_disk(disk)
         return gaps.largest_gap(disk)
 
-    def guided_direct(self, disk):
-        gap = self.setup_disk_for_guided(disk)
+    def guided_direct(self, disk, mode=None):
+        gap = self.setup_disk_for_guided(disk, mode)
         spec = dict(fstype="ext4", mount="/")
         self.create_partition(device=disk, gap=gap, spec=spec)
 
-    def guided_lvm(self, disk, lvm_options=None):
-        gap = self.setup_disk_for_guided(disk)
+    def guided_lvm(self, disk, mode=None, lvm_options=None):
+        gap = self.setup_disk_for_guided(disk, mode)
         gap_boot, gap_rest = gap.split(sizes.get_bootfs_size(gap.size))
         spec = dict(fstype="ext4", mount='/boot')
         self.create_partition(device=disk, gap=gap_boot, spec=spec)
@@ -198,7 +199,7 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
                         'password': choice.password,
                         },
                     }
-            self.guided_lvm(disk, lvm_options)
+            self.guided_lvm(disk, lvm_options=lvm_options)
         else:
             self.guided_direct(disk)
 
@@ -459,19 +460,36 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
                 continue
             break
 
+    def run_guided(self, layout):
+        guided_method = getattr(self, "guided_" + layout['name'])
+        mode = layout.get('mode', 'reformat_disk')
+        self.validate_layout_mode(mode)
+
+        if mode == 'reformat_disk':
+            match = layout.get("match", {'size': 'largest'})
+            disk = self.model.disk_for_match(self.model.all_disks(), match)
+            if not disk:
+                raise Exception("autoinstall cannot configure storage "
+                                "- no disk found large enough for install")
+        elif mode == 'use_gap':
+            gap = gaps.largest_gap(self.model.all_disks())
+            if not gap:
+                raise Exception("autoinstall cannot configure storage "
+                                "- no gap found large enough for install")
+            # This is not necessarily the exact gap to be used, as the gap size
+            # may change once add_boot_disk has sorted things out.
+            disk = gap.device
+        guided_method(disk=disk, mode=mode)
+
+    def validate_layout_mode(self, mode):
+        if mode not in ('reformat_disk', 'use_gap'):
+            raise ValueError(f'Unknown layout mode {mode}')
+
     @with_context()
     def convert_autoinstall_config(self, context=None):
         log.debug("self.ai_data = %s", self.ai_data)
         if 'layout' in self.ai_data:
-            layout = self.ai_data['layout']
-            meth = getattr(self, "guided_" + layout['name'])
-            disk = self.model.disk_for_match(
-                self.model.all_disks(),
-                layout.get("match", {'size': 'largest'}))
-            if not disk:
-                raise Exception("autoinstall cannot configure storage "
-                                "- no disk found large enough for install")
-            meth(disk)
+            self.run_guided(self.ai_data['layout'])
         elif 'config' in self.ai_data:
             self.model.apply_autoinstall_config(self.ai_data['config'])
             self.model.grub = self.ai_data.get('grub')
diff --git a/subiquity/server/controllers/tests/test_filesystem.py b/subiquity/server/controllers/tests/test_filesystem.py
index 41307a92..f73e33c4 100644
--- a/subiquity/server/controllers/tests/test_filesystem.py
+++ b/subiquity/server/controllers/tests/test_filesystem.py
@@ -15,6 +15,8 @@
 
 from unittest import mock, TestCase
 
+from parameterized import parameterized
+
 from subiquity.server.controllers.filesystem import FilesystemController
 
 from subiquitycore.tests.util import run_coro
@@ -103,3 +105,21 @@ class TestGuided(TestCase):
         self.assertEqual(None, d1p1.mount)
         self.assertEqual('/', d1p2.mount)
         self.assertEqual(d1p1.size + d1p1.offset, d1p2.offset)
+
+
+class TestLayout(TestCase):
+    def setUp(self):
+        self.app = make_app()
+        self.app.opts.bootloader = 'UEFI'
+        self.app.report_start_event = mock.Mock()
+        self.app.report_finish_event = mock.Mock()
+        self.fsc = FilesystemController(app=self.app)
+
+    @parameterized.expand([('reformat_disk',), ('use_gap',)])
+    def test_good_modes(self, mode):
+        self.fsc.validate_layout_mode(mode)
+
+    @parameterized.expand([('resize_biggest',), ('use_free',)])
+    def test_bad_modes(self, mode):
+        with self.assertRaises(ValueError):
+            self.fsc.validate_layout_mode(mode)