Project Server - Open site in SharePoint Designer

If you want to edit some of the forms of SharePoint lists in your Project Server (like Project Server Worklow Tasks or Project Server Workflow History, etc), logically, you would think of opening your PWA site in SharePoint Designer and edit it there.
But, you'd be wrong.

Opening Project Server web site in SharePoint has been disabled, and if you try to open in in Designer, you'll get following error:

“Web Site Editing is Disabled. This web site has been configured to disallow editing with SharePoint Designer. Contact your web site administrator for more information.”

To resolve this you just need to remove the DisableWebDesignFeatures="wdfopensite" from the ONET.XML File.

You can do that by following procedure:

1. Open Windows Explorer and go to folder (<site_name> is usually PWA):

C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\SiteTemplates\<site_name>\XML

2. Make backup copy of  ONET.xml file in that folder before making changes in that file.

3. Find text DisableWebDesignFeatures="wdfopensite" in ONET.xml and remove it from file.

4. Save changes and restart IIS.