I am building a detached garage with future living space on the second floor. I wish to have a toilet and sink in the garage, and also rough in the needed drains/vents in the slab for a full bathroom upstairs. Can I utilize the same drains for both toilets and sinks? How do I ensure that I have the right set up for the future upstairs bathroom? Should I use separate drains and combine them under the slab? Thanks for any help you can provide.

Use these simple rules
1- a toilet must be a minimum of 3" pipe
2- a toilet should not flush by another trap unless the other trap is on a vented line
3- a system must be separate for separate floor levels and each floor level must be vented
4- a fixture or appliance with a motor driven drain can not enter the system within 10 feet downstream or any part upstream of a wet vented toilet [washing machine, dishwasher, disposer]
5- a lavatory or sink may be piped with a minimum of a 2" line with no vent if no motor is used
6- an example of a wet vent is a lavatory or shower or tub or sink is vented then connected to a system and flowing past a non vented toilet. The piping containing waste water only is considered a wet vent. Waste water has no 'stuff' in it. Soil water has 'stuff' in it from a toilet. The toilet must be within 10 feet of this wet vent line.