The present invention is a cartridge underbody attachment for a single control faucet. The attachment generally comprises a cartridge seat, valve tubes, and tube fittings. The cartridge seat is supported within the spout body of a faucet. The spout body comprises a sideways extending port having a notch and ledge formed therein. The cartridge seat is positioned in the spout body. The cartridge seat generally comprises a backplate having defined apertures, and a manifold that is part of the rear surface of backplate. Valve tubes soldered within the manifold are in fluid communication with the apertures. A pin on the backplate interfaces with the notch in the spout body to align and position the cartridge seat within the spout body. A cartridge support housing in combination with the port ledge of the spout body secure the cartridge underbody attachment to the spout body.