I want to add a shower, sink and toilet to my unattached garage. I need to run septic line approx 100ft to get to septic tank. Is that too far?

No, it's not too far, but you will definitely need to plan on adding a cleanout at between 50-75 feet out, depending on your local code requirements.

You also need to be able to have proper pitch for the pipe. Here, if you run 4" pipe you only need an 1/8" pitch per foot of pipe. If you run 3" pipe you will need a 1/4" pitch per foot of pipe.

Now, are you sure your septic tank can handle another bathroom? If not, you may want to consult with your local board of health... just to make sure.

In terms of the actual plumbing in the garage do you know if they allow WET VENTING in your area? If so, this is usually a time/money saver we should discuss more.

Check into wet venting...

Mark

PS: Any chance the garage run of pipe could be shorter if you connected it into the main house plumbing system? Let me know.

Im actually considering making it a pool house for the pool, so It wouldn`t be used too often. However at that distance I`m thinking the input into the spetic tank will be too low. Im in TN. I may need to consider just building a pool house instead.