Finally putting a bathroom in the basement and here is what I'll be submitting to the builing inspector.

I've got one open space in my panel and plan on putting a 20/20 tandem breaker in with 12/3. One of the circuits will go to a GFCI on the wall behind the toilet to run the pump (9amps) and then to an outlet by sink. The other circuit will run a floor warming system (5 amps) and the jacuzzi tub (10amps). I will be grabbing power from an adjacent room for the 3 wall sconces and sink light and from another adjacent room for the ceiling fan. It might be hard to visualize but do you guys see any problem with this?

Since you only have one full size slot in the breaker, using a tandem breaker for the additional circuits using #12-3 will only work by converting at least two other full size breakers to tandem.

The one slot you have only will allow connection to one leg of the panel, and the new tandem must connect to both legs, when using two circuits and a shared neutral in a #12-3 cable.

Re-arranging the breakers to get one tandem onto both legs may be a challenge. You can use the new tandem in the one available slot ONLY if you use #12-2 cable for each new circuit feeder.

OK, found out I may have jumped the gun a bit as the whirlpool tub needs a dedicated circuit with a gfci breaker so does the floor warmer so I'm going to have to do some juggling. Maybe put a couple tandems in (I can put up to three in my box). Thanks for the advice.