Connecting new bathroom to existing stack

I have a ranch house with 1 bathroom. 3" main stack passes about 8-9 feet away from the toilet in the new bathroom.

Is there any reason that I can't wye off that stack(above the height of the old bathroom) and go horizontal in a ceiling soffit on first floor and pick up the new drain..

because of the distance to the stack I would of course re-vent all those fixtures.

Normally I think this would be okay.. but is there anything I should review with the existing bathroom that would disqualify this...

I just wanted to follow up and see if anyone had any comments on this.

If I were to tie in to the main stack with a second floor bathroom, can I tie in above the level of the 1st floor bathroom or would I need to go down with a new pipe and get below the level of the 1st floor basement?

Depending on the code used where you live, there may be a horizontal to vertical max ratio in the vent line. Regardless, any horizontal sections of the vent need to still have slope to it so in case of condensation or rain or whatever getting in there must be able to drain. This will bump you back up towards the top. A sketch of the distances and layout would help.

I will try and put together a picture a little later.

The horizonal is actually the drain for the toilet to reach the main stack. I measured it today, toilet is about 7' away from the main stack. So at a 1/4" per foot that is 2" of pitch as a go across the soffit. I have plenty of room above the cabinets to make a soffit that big.

My issue was more along the lines of can a 3" stack that is acting as a vent for a 1st floor bath also also act as drain for a second floor bath? The kitchen sink is seperatly vented but as far as I can tell the bath is vented by the main stack only(which makes sense since the stack was located about 2' from the 1st floor toilet and sink).

I would have to check the math but I wouldnt think that this would exceed the capacity