You should avoid installing most countertop dishwashers underneath the sink. The hoses and connections of these models are not designed for permanent installation and may be subject to leaks or bursting. However, you can install countertop dishwashers approved by the manufacturer for built-in use under the sink. The COSTWAY FP10017 Countertop Dishwasher is one model that includes instructions for permanent installation in the user manual. If you are unsure about a countertop dishwasher's specifications or plumbing needs, consult with a plumber before proceeding with the built-in installation.