Adding second bathroom

Alvis

Hi all...
The plan here is to tie the new bathroom drains into the existing drain, which then dumps into a septic tank and drain field.

So... the question is... can I tap into the existing Y cleanout under the house for the new drain (4")? Or would I have to dig up the existing pipe and put a 4" Y in?

Also, with regards to a vent pipe, can I tie the new vent pipes together (in the very tight attic) and run it horizontal (10 feet) into the existing (3"?) vent pipe, or does the new room need it's own vent?

More questions coming I'm sure.

Any replys welcome... tia... Al