Levelable cassette of nominal size 9, decoupled from the floor, with cord outlet, for the installation of installation devices in dry-maintained floors indoors, for installation in underfloor boxes UZD250-3 or UZD350-3, each in conjunction with a mounting cover DUG../9 or in UGD250-3/9 or UGD350-3/9. Installation is carried out using leveling angles (order separately) in the aluminum frame profile of the underfloor boxes.