diff --git a/3d-viewer/3d_cache/3d_cache.cpp b/3d-viewer/3d_cache/3d_cache.cpp
index fdb2855d8f..a41c727f67 100644
--- a/3d-viewer/3d_cache/3d_cache.cpp
+++ b/3d-viewer/3d_cache/3d_cache.cpp
@@ -47,6 +47,7 @@
 #include "sg/scenegraph.h"
 #include "plugins/3dapi/ifsg_api.h"
 
+#include <common.h>     // For ExpandEnvVarSubstitutions
 #include <filename_resolver.h>
 #include <paths.h>
 #include <pgm_base.h>
diff --git a/common/paths.cpp b/common/paths.cpp
index 60c82a1ccf..61e1069f0b 100644
--- a/common/paths.cpp
+++ b/common/paths.cpp
@@ -20,6 +20,7 @@
 #include <wx/filename.h>
 #include <wx/stdpaths.h>
 #include <wx/string.h>
+#include <wx/utils.h>
 
 #include <kiplatform/environment.h>
 #include <paths.h>
diff --git a/common/settings/settings_manager.cpp b/common/settings/settings_manager.cpp
index f297419c90..6224f32a16 100644
--- a/common/settings/settings_manager.cpp
+++ b/common/settings/settings_manager.cpp
@@ -22,6 +22,7 @@
 #include <wx/debug.h>
 #include <wx/dir.h>
 #include <wx/filename.h>
+#include <wx/snglinst.h>
 #include <wx/stdpaths.h>
 #include <wx/utils.h>
 
@@ -31,6 +32,7 @@
 #include <gestfich.h>
 #include <kiplatform/environment.h>
 #include <kiway.h>
+#include <lockfile.h>
 #include <macros.h>
 #include <paths.h>
 #include <project.h>
@@ -757,6 +759,15 @@ bool SETTINGS_MANAGER::LoadProject( const wxString& aFullPath, bool aSetActive )
     if( m_projects.count( fullPath ) )
         return true;
 
+    bool readOnly = false;
+    std::unique_ptr<wxSingleInstanceChecker> lockFile = ::LockFile( fullPath );
+
+    if( !lockFile )
+    {
+        wxLogTrace( traceSettings, "Project %s is locked; opening read-only", fullPath );
+        readOnly = true;
+    }
+
     // No MDI yet
     if( aSetActive && !m_projects.empty() )
     {
@@ -773,7 +784,12 @@ bool SETTINGS_MANAGER::LoadProject( const wxString& aFullPath, bool aSetActive )
     bool success = loadProjectFile( *project );
 
     if( success )
-        project->SetReadOnly( project->GetProjectFile().IsReadOnly() );
+    {
+        project->SetReadOnly( readOnly || project->GetProjectFile().IsReadOnly() );
+
+        if( lockFile )
+            m_project_lock.reset( lockFile.release() );
+    }
 
     m_projects_list.push_back( std::move( project ) );
     m_projects[fullPath] = m_projects_list.back().get();
@@ -825,6 +841,9 @@ bool SETTINGS_MANAGER::UnloadProject( PROJECT* aProject, bool aSave )
     // Remove the reference in the environment to the previous project
     wxSetEnv( PROJECT_VAR_NAME, "" );
 
+    // Release lock on the file, in case we had one
+    m_project_lock = nullptr;
+
     if( m_kiway )
         m_kiway->ProjectChanged();
 
diff --git a/eeschema/dialogs/dialog_plot_schematic.cpp b/eeschema/dialogs/dialog_plot_schematic.cpp
index 38a7531892..9166345a5e 100644
--- a/eeschema/dialogs/dialog_plot_schematic.cpp
+++ b/eeschema/dialogs/dialog_plot_schematic.cpp
@@ -25,6 +25,7 @@
  */
 
 #include <bitmaps.h>
+#include <common.h>     // For ExpandEnvVarSubstitutions
 #include <dialogs/wx_html_report_panel.h>
 #include <dialog_plot_schematic.h>
 #include <eeschema_settings.h>
diff --git a/eeschema/dialogs/panel_sym_lib_table.cpp b/eeschema/dialogs/panel_sym_lib_table.cpp
index da92e2a175..7613441da9 100644
--- a/eeschema/dialogs/panel_sym_lib_table.cpp
+++ b/eeschema/dialogs/panel_sym_lib_table.cpp
@@ -22,6 +22,7 @@
 #include <set>
 #include <wx/regex.h>
 
+#include <common.h>     // For ExpandEnvVarSubstitutions
 #include <project.h>
 #include <panel_sym_lib_table.h>
 #include <lib_id.h>
diff --git a/include/settings/settings_manager.h b/include/settings/settings_manager.h
index 016493c8f3..9690e10324 100644
--- a/include/settings/settings_manager.h
+++ b/include/settings/settings_manager.h
@@ -22,7 +22,7 @@
 #define _SETTINGS_MANAGER_H
 
 #include <typeinfo>
-#include <common.h> // for wxString hash
+#include <core/wx_stl_compat.h> // for wxString hash
 #include <settings/color_settings.h>
 
 class COLOR_SETTINGS;
@@ -31,6 +31,7 @@ class KIWAY;
 class PROJECT;
 class PROJECT_FILE;
 class REPORTER;
+class wxSingleInstanceChecker;
 
 
 class SETTINGS_MANAGER
@@ -425,6 +426,9 @@ private:
     /// Loaded project files, mapped according to project full name
     std::map<wxString, PROJECT_FILE*> m_project_files;
 
+    /// Lock for loaded project (expand to multiple once we support MDI)
+    std::unique_ptr<wxSingleInstanceChecker> m_project_lock;
+
     static wxString backupDateTimeFormat;
 };