diff --git a/docs/GUIDELINES.md b/docs/GUIDELINES.md
index 5e6666e..5fcd934 100644
--- a/docs/GUIDELINES.md
+++ b/docs/GUIDELINES.md
@@ -9,10 +9,19 @@ type: default
 - Every build records the source PBF's date. An artifact whose provenance is
   unknown is an artifact to rebuild.
 
-## 2. Build the admin database first
+## 2. Build the admin database first, and from the *full* extract
 - `valhalla_build_admins` is what determines driving side, and it is a separate
   step *before* the tile build.  
-- Skip it and Valhalla routes and narrates as though Japan drove on the right.
+- **Always build it from `japan-latest.osm.pbf`, never a sub-region.** Driving side
+  is a country-level property. A Shikoku extract yields only `admin_level=4`
+  prefecture rows, no country record, and `drive_on_right=1` on every one —
+  measured, not theorised.  
+- The admin build and the tile build are separate invocations against the same
+  config, so routing tiles may still come from a small regional extract while the
+  admin database comes from the whole country.  
+- **Verify, do not assume.** Every build checks that a country-level row exists and
+  reports `drive_on_right=0` before the tiles are considered usable. This failure
+  is silent otherwise: routing simply comes out mirrored.
 
 ## 3. Tiles ship as a directory, not a tar
 - Once for RAM on an 8 GB board, once because `rsync` deltas a tree of changed