Ready-to-install duct extension unit with a device installation cover for installing an installation unit of nominal size 9, consisting of side profiles with attached flexible side wall made of metal mesh, floor covering attachment profiles, duct cover with blindly closed installation opening and a duct end piece. The scope of delivery includes 3 height-adjustable fastening brackets, 3 screed anchors, 1 cover joint support (without leveling unit - please order separately) and 1 kit for angle branch.