I am adding a 2nd story powder room with a toilet and sink to my existing town-home.

I am draining the two fixtures with a 3 inch line. The drain line runs down an exterior wall (on the inside of it), and then exits just above ground level. From that point, it is about a 7 foot run to my outside, 3 inch cleanout.

Not sure how the cleanout line is tied into the horizontal exit line below.

Can I "T" into this vertical cleanout line?

Or must I dig down to the horizontal line that the "vertical cleanout line" connects into? If I have to T into the lower horizontal line, what is the best way to do this? One of those rubber connecting T's or something better?

Thank you.