I have had an above garage extension with bathroom. The architect planned it that a new manhole would be dug and we would tee into the main drain approx 15ft away which is shared with neighbours. Not sure where it is exactly and we know it is 5ft deep, I've looked into the possibility of running it directly below into the garage against the wall  then horizontal across the wall for 3 feet, then knock a hole through the wall into the downstairs toilet. Run it down the wall in there next to the toilet and get the plumber to rejig the soil pipe in there. Any thought on which is best and cost for new drain outside. I guess the inside option will be 3 to£400 going off previous work. 

Also will the BI be ok with an internal drain do this way. I know it needs fireproofing.

Thank in advance for any thought

Does the GF WC exit into a vented soil stack?

Hi mate. It is vented to atmos  via the stack on our other bathroom they both go to the same chamber.