Currently, OsmAnd checks the free space against the cumulated size of the downloaded files.

This works well with new downloads, but is irritating with updates.
Properly calculate the changed disk usage and max temp usage and show it to the user.
Don't allow the download if the free space will not be sufficient.

OsmAnd/res/values/strings.xml

@@ -9,6 +9,15 @@
          3. All your modified/created strings are in the top of the file (to make easier find what\'s translated).
     PLEASE: Have a look at http://code.google.com/p/osmand/wiki/UIConsistency, it may really improve your and our work  :-)  Thx - Hardy
     -->
+    <string name="download_files_error_not_enough_space">Not enough space!
+        This would need {3} MB temporarily and {1} MB permanently.
+        Currently, there are only {2} MB available.</string>
+    <string name="download_files_question_space_with_temp">Really download {0} file(s)?
+        This needs {3} MB temporarily and {1} MB permanently?
+        Currently, there are {2} MB available.</string>
+    <string name="download_files_question_space">Really download {0} file(s)?
+        This needs {1} MB permanently?
+        Currently, there are {2} MB available.</string>
     <string name="show_map_markers_topbar">Show Map markers topbar</string>
     <string name="map_marker_1st">First Map marker</string>
     <string name="map_marker_2nd">Second Map marker</string>
@@ -1612,7 +1621,6 @@ If you need help with OsmAnd application, please contact our support team: suppo
     <string name="osmand_service">Background mode</string>
     <string name="osmand_service_descr">OsmAnd runs in background while screen is off</string>
     <string name="download_files_not_enough_space">There is not enough free space to download %1$s MB (free: %2$s).</string>
-    <string name="download_files_question_space">Free space now {2} MB! Download {0} file(s) ({1} MB)?</string>
     <string name="use_transparent_map_theme">Transparent theme</string>
     <string name="native_library_not_supported">Native library is not supported on this device.</string>
     <string name="init_native_library">Initializing native library…</string>

OsmAnd/src/net/osmand/plus/download/DownloadValidationManager.java

@@ -19,6 +19,7 @@ import net.osmand.plus.R;
 import net.osmand.plus.Version;
 import net.osmand.plus.download.DownloadIndexesThread.DownloadEvents;
 
+import java.io.File;
 import java.text.MessageFormat;
 
 public class DownloadValidationManager {
@@ -46,23 +47,50 @@ public class DownloadValidationManager {
 		return app;
 	}
 
+	private long getExistingFileSize(File file) {
+		if (file != null) {
+			if (file.canRead()) {
+				return file.length();
+			}
+		}
+		return 0;
+	}
+
 	public void downloadFilesCheck_3_ValidateSpace(final FragmentActivity context, final IndexItem... items) {
-		long szLong = 0;
+		long szChangeLong = 0;
+		long szMaxTempLong = 0;
 		int i = 0;
 		for (IndexItem es : downloadThread.getCurrentDownloadingItems()) {
-			szLong += es.contentSize;
+			final long szExistingLong = getExistingFileSize(es.getTargetFile(getMyApplication()));
+			long change = es.contentSize - szExistingLong;
+			szChangeLong += change;
+			if (szExistingLong > szMaxTempLong) szMaxTempLong = szExistingLong;
 			i++;
 		}
 		for (IndexItem es : items) {
-			szLong += es.contentSize;
+			final long szExistingLong = getExistingFileSize(es.getTargetFile(getMyApplication()));
+			long change = es.contentSize - szExistingLong;
+			szChangeLong += change;
+			if (szExistingLong > szMaxTempLong) szMaxTempLong = szExistingLong;
 			i++;
 		}
-		double sz = ((double) szLong) / (1 << 20);
+		double szChange = ((double) szChangeLong) / (1 << 20);
+		double szMaxTemp = szChange + ((double) szMaxTempLong) / (1 << 20);
+
 		// get availabile space
 		double asz = downloadThread.getAvailableSpace();
-		if (asz != -1 && asz > 0 && sz / asz > 0.4) {
+		if (asz != -1 && asz > 0 && (szMaxTemp > asz)) {
 			AlertDialog.Builder builder = new AlertDialog.Builder(context);
-			builder.setMessage(MessageFormat.format(context.getString(R.string.download_files_question_space), i, sz, asz));
+			builder.setMessage(MessageFormat.format(context.getString(R.string.download_files_error_not_enough_space), i, szChange, asz, szMaxTemp));
+			builder.setNegativeButton(R.string.shared_string_no, null);
+			builder.show();
+		} else if (asz != -1 && asz > 0 && (szChange / asz > 0.8 || szMaxTemp / asz > 0.9)) {
+			AlertDialog.Builder builder = new AlertDialog.Builder(context);
+			if (szChange != szMaxTemp) {
+				builder.setMessage(MessageFormat.format(context.getString(R.string.download_files_question_space_with_temp), i, szChange, asz, szMaxTemp));
+			} else {
+				builder.setMessage(MessageFormat.format(context.getString(R.string.download_files_question_space), i, szChange, asz));
+			}
 			builder.setPositiveButton(R.string.shared_string_yes, new DialogInterface.OnClickListener() {
 				@Override
 				public void onClick(DialogInterface dialog, int which) {