[rbfoytik]

I built my house ten years ago and had rough ins done for a lav, stool and shower in the basement. The lav and stool have been hooked up for 2 years and work great. However, instead of installing a shower I want to install a kitchen sink with a garbage disposal and dishwasher. The existing stub has a trap installed under the floor. Can I hook up the sink to this drain and use the existing trap, should I double trap or do I need to remove the underground trap and have traps under the sink?

[speedball1]

I built my house ten years ago and had rough ins done for a lav, stool and shower in the basement. The lav and stool have been hooked up for 2 years and work great. However, instead of installing a shower I want to install a kitchen sink with a garbage disposal and dishwasher. The existing stub has a trap installed under the floor. Can I hook up the sink to this drain and use the existing trap, should i double trap or do i need to remove the underground trap and have traps under the sink?

First off, you may not double trap a fixture, so discount that.
The plumbing inspector and most plumbers will tell you to rip up the floor and run up a stub out for the sink and trap and revent it back to the lav vent.
However, since I have a hunch you won't be pulling a permit for this remodel I would use the existing shower trap. Your shower should have been wet vented if you tied the drain back to the lavatory drain so that should take care of the vent. For added insurance I'd install a AAV, Air Addimitence Valve or cheater vent off a tee just behind where the drain goes out of the disposal down to the trap. That leaves connecting up the drain. After removing the shower drain you should be left with a 2" stub up out of the floor you may couple on a drain line to the stub up using a 2" No-Hub Clamp, (see image).
And pipe to the AAV tee from there. Good luck, Tom