diff --git a/org.eclipse.xtext.builder/src/org/eclipse/xtext/builder/impl/BuilderStateDiscarder.java b/org.eclipse.xtext.builder/src/org/eclipse/xtext/builder/impl/BuilderStateDiscarder.java
index 5437f96446..37b60a6bc5 100644
--- a/org.eclipse.xtext.builder/src/org/eclipse/xtext/builder/impl/BuilderStateDiscarder.java
+++ b/org.eclipse.xtext.builder/src/org/eclipse/xtext/builder/impl/BuilderStateDiscarder.java
@@ -8,6 +8,9 @@
  *******************************************************************************/
 package org.eclipse.xtext.builder.impl;
 
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.MethodType;
 import java.util.Map;
 
 import org.apache.log4j.Logger;
@@ -34,12 +37,34 @@
 public class BuilderStateDiscarder {
 
 	private static final Logger logger = Logger.getLogger(BuilderStateDiscarder.class);
+	private static final MethodHandle InternalBuilder_getLastBuiltTree;
 
+	static {
+		try {
+			@SuppressWarnings("restriction")
+			MethodHandles.Lookup lookup = MethodHandles.privateLookupIn(
+					org.eclipse.core.internal.events.InternalBuilder.class, MethodHandles.lookup());
+			@SuppressWarnings("restriction")
+			MethodHandle handle = lookup.findVirtual(
+					org.eclipse.core.internal.events.InternalBuilder.class, 
+					"getLastBuiltTree", 
+					MethodType.methodType(org.eclipse.core.internal.watson.ElementTree.class));
+			InternalBuilder_getLastBuiltTree = handle;
+		} catch (ReflectiveOperationException e) {
+			throw new ExceptionInInitializerError(e);
+		}
+	}
+	
 	/**
 	 * Returns true, if the given builderArguments indicate that we should discard the built state
 	 * for the given projects.
 	 */
 	public boolean forgetLastBuildState(Iterable<IProject> toUpdate, Map<String, String> builderArguments) {
+		/*
+		 * Implementation note:
+		 * This is executed with the Workspace.lock being acquired, but the tree might be locked for modifications,
+		 * so we may not directly touch a project.
+		 */
 		if (canHandleBuildFlag(builderArguments)) {
 			for (IProject project : toUpdate) {
 				XtextBuilder builder = BuildManagerAccess.findBuilder(project);
@@ -61,7 +86,8 @@ public boolean forgetLastBuildState(Iterable<IProject> toUpdate, Map<String, Str
 	protected void touchProject(IProject project) {
 		Job touchJob = Job.create("Touch project " + project.getName(), progressMonitor -> {
 			try {
-				if (project.isAccessible()) {
+				// Check if there is already a known built tree before touching the project
+				if (project.isAccessible() && !hasBeenBuilt(project)) {
 					project.touch(progressMonitor);
 				}
 				return Status.OK_STATUS;
@@ -77,6 +103,17 @@ protected void touchProject(IProject project) {
 
 	}
 
+	protected boolean hasBeenBuilt(IProject project) {
+		@SuppressWarnings("restriction")
+		org.eclipse.core.internal.events.InternalBuilder builder = BuildManagerAccess.findBuilder(project);
+		try {
+			return InternalBuilder_getLastBuiltTree.invoke(builder) != null;
+		} catch (Throwable e) {
+			logger.error("Failed to dermine if the project was already built. Assuming it wasn't built yet", e);
+			return  false;
+		}
+	}
+
 	protected boolean canHandleBuildFlag(Map<String, String> builderArguments) {
 		return IBuildFlag.FORGET_BUILD_STATE_ONLY.isSet(builderArguments)
 				|| IBuildFlag.RECOVERY_BUILD.isSet(builderArguments);