We have a 1/2 bath, tool shed on the back wall. Going to knock down the wall and make a full bath add a tub and shower. My questions are, can I tap into the water going to the sink, use pipes through the wall for the tub and shower? Will this provide enough water pressure. I understand all three could not be running simultaneously. Second, the tub and shower will be located on the back wall of the house about ten foot from the septic tank. Can I run a new drainage pipe for the tub and shower going out the back wall, or do I have to tie into the existing drainage pipe? If I can run a new pipe, do I have to drain into the septic tank? Any suggestions or ideas would be greatly appreciated.

1. You can get away with 1/2" line for full bath. I have seen it many times before and nobody complained. However, if you have access to 3/4" cold water supply - than run 3/4" line.
2. You can run separate drains to the outside of house. However, you will probably want to connect with existing main sewer somewhere on the outside before it connects to septic. Otherwise, you would have to drill new hole in the correct side of septic tank.
3. In rural areas, people run tub/shower/sink drains to the garden and let the discharged "gray" water to irrigate their gardens. However, this approach is not recommended if you live in more populated area