diff --git a/airline-web/app/controllers/AirlineApplication.scala b/airline-web/app/controllers/AirlineApplication.scala
index b299de569..ae3712163 100644
--- a/airline-web/app/controllers/AirlineApplication.scala
+++ b/airline-web/app/controllers/AirlineApplication.scala
@@ -270,6 +270,15 @@ class AirlineApplication @Inject()(cc: ControllerComponents) extends AbstractCon
       return Some("Not enough cash to build/upgrade the base")
     }
 
+    val existingBase = airline.getBases().find(_.airport.id == targetBase.airport.id)
+    if (existingBase != undefined) {
+      if (targetBase.scale != existingBase.scale + 1) {
+        return Some(s"Can only upgrade base to level ${existingBase.scale + 1}")
+      }
+    } else if (targetBase.scale != 1) {
+      return Some(s"Base does not exist yet, can only build level 1 base")
+    }
+
     if (targetBase.scale == 1) { //building something new
       if (airline.getHeadQuarter().isDefined) { //building non-HQ
         AllianceSource.loadAllianceMemberByAirline(airline).filter(_.role != AllianceRole.APPLICANT).foreach { allianceMember =>