Could use some help with existing drain layout and where to tie in new line

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Coreman73

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I'm considering building a bathroom in my attic as part of an attic remodel and want to connect the new drain line to the downstairs bathroom main stack, which is below my slab. I've checked the city, county and builder and nobody has a blueprint or plan for my home to show the exact layout.

I have a pretty good idea but was hoping for someone to provide a best guess on how things might be hooked up. I've attached a photo of this downstairs bathroom (excuse the mess). The arrows show the approximate location of everything. The wall that the sink and toilet are on is an external wall. The clean out is accessed about 12" above the slab on this external wall. The sink and toilet both vent to the 2" vent. The tub vents separately along the small partition wall opposite.

I could use some advice where I would tie the new 3" waste stack from the attic into the main stack beneath the slab.
 

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Coreman73

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I decided to put together a rough diagram showing what I believe my existing system on the bottom floor (under slab) looks like. I've also drawn in what I'd like to do for the new attic bathroom. Please provide any advice. It's attached.

The two spots with question marks are because I don't know exactly where to run them. For the 2" attic bathroom vent, there's likely not enough physical space to tie in to the existing 2" vent due to small gap between the top plate of wall and bottom of roof. As an alternative, could I adapt the existing 1-1/2" vent to 2" and tie in there?

As for the 3" attic bathroom main drain line, where should I tie in to the main stack under the slab?
 

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Coreman73

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Thanks Terry and also to Reach4 who provided me with the same diagram in another thread but for whatever reason it didn't sink in until now. It takes me awhile sometimes for things to make sense.

On the drawing I showed above, I realize code does not allow me to vent the new attic fixtures in to the existing 2" vent coming from downstairs due to height. Since that is the case, it appears I only have two choices. I can either penetrate the roof to create another 2" vent OR tie in to the existing 1-1/2" vent pipe coming up from the downstairs bathtub.

Is the second option allowed? In it's current size obviously not but could it be adapted to 2" at the point I tie in to it?

Finally, would the new main drain line from the attic be tied in at the point between where the clean out and sink drains tie in to the main stack beneath the slab? The above diagram seems to indicate that.
 
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