I have a basement bathroom and an upstairs bathroom... speedball1 you stated in one of your questions answered

"Yes, but you must realize that once you drain the fixtures from the upstairs bath into it that it will no longer be a vent from that point down . It then becomes a stack and every fixture below,( the basement half bath), and will not be able to use this as a vent. They will have to be vented out the roof or revented back into a dry vent."

... my full bathroom upstairs uses the half bath in the basements vent... i've been living here for five years and i noticed that the basement half bath in basement is not revented back into a dry vent above the full bath upstairs... is this a major problem... the reason i want to know is that i'm thingking of adding an second floor bathroom and i already have this problem?. if i don't add another bathroom should i try to vent the half bath in the basement... i would have to demo a lot of tile that is already there that i really don't want to.

A "stack" is not a "vent". Chapter 14, Section 1411.2 of the Standard Plumbing Code titled "Different Levels" reads, "If any stack has fixtures entering at different levels, (floors), the fixtures, other then the the fixture entering at the highest level must be vented or revented into a dry vent."
All this means is that you can not discharge a major fixture , such as a toilet past a unvented minor one, such as a tub or shower.
However, ( and I'm going out on a limb here) this will present no problem to you unless you pull a permit on the proposed bathroom addition. If the inspector finds a code violation in your house you will be forced to bring it up to code.
Code always errs on the side of caution. Code takes a "worst case scenario" and writes to that.
Practically speaking, (and here I am out on a limb again,) will your basement bath vent through the stack that services the full bath? Yes it will. Is it in code? No, it isn't.
Speaking as a home owner and not as a plumber If this were my house I would install the second floor bathroom and run a vent up to the attic and revent into a existing dry or roof vent and not tear out walls and tile attempting to vent a bathroom group that has operated without problems for over five years.
Speaking as a plumber I would vent the basement bath group to code.
Your choice! Good luck, Tom

So if I don't what kind of problem is it major... what could happen if I don't bring it up to code ? What are the consequences... what could happen ?anything major? WHAT IS THE WORST CASE SCENARIO ?

Can the basement half bath(toilet and sink) use studor vents... IF CODE ALLOWED IT... THE BASEMENT HALF BATH(TOILET AND SINK)IS NOT VENTED PROPERLY BECAUSE IT USES THE FIRST FLOOR ABOVE IT'S STACK... so again CAN THE BASEMENT TOILET AND SINK USE STUDOR VENTS EVEN IS THE IS A BATHROOM GROUP ONE FLOOR ABOVE IT?

I GUESS I WAS READING SOMETHING ABOUT 6" above highest rim something... I really don't know what they were talking about.

"So if I don't what kind of problem is it major ... what could happen if i don't bring it up to code ? what are the consequences ... what could happen ?anything major? WHAT IS THE WORST CASE SCENARIO?"
I've answered this when I said, "Will your basement bath vent through the stack that services the full bath? Yes it will. Is it in code? No, it isn't."
The "worst case scenario" comes into play if a inspector red tags your house as unsafe to habitant. You will then have no option but to bring your house up to code.

"can the basement half bath(toilet and sink) use studor vents ... IF CODE ALLOWED IT?" Yes, if code allows. However, if you disconnect the vent pipe from the lavatory that you've connected to the stack and install a AAV then a toilet vent won't be needed as it will vent through the lavatory.

"I GUESS I WAS READING SOMETHING ABOUT 6" above highest rim something... I really don't know what they were talking about."
All this means is that you wouldn't have to run the basement vent out the roof if you decided to vent. You could just run up to, say, a lavatory vent, if you tied the basement vent into the lavatory vent 6" above where the lavatory would flood the floor if it ran over. I just hope I didn't confuse you farther.
Will a permit be pulled for the remodel? Regards, Tom

Yes a permit will be pulled.

You stated... if you disconnect the vent pipe from the lavatory that you've connected to the stack and install a AAV then a toilet vent won't be needed as it will vent through the lavatory.. . (THE TOILET VENT WON'T BE NEEDED)|

Scenario the toilet and sink are right next to each other which is RIGHT NEXT TO THE STACK

So the lavatory 2" pipe drains into the side of the closet elbow of the toilet (and lavatory has its own vent through the roof)... then the toilet closet elbow goes directly into the main stack (which I thought is it's vent)... so what you are saying is (correct me if I'm wrong) the toilet and lavatory can use the lavatory vent now and the toilet does not need to have a separate vent?. they can share the sink vent?. even though the vent that the toilet would be sharing is also being used for the lavatory drain... hope I didn't confuse you

You say," hope i didn't confuse you." Oh boy! Am I confused!

"MY FULL BATHROOM UPSTAIRS USES THE HALF BATH IN THE BASEMENTS VENT"
This is what you stated in a earlier post. This says to me that the basement group is using the stack as a vent. This is discharging a major fixture past a unvented minor one. A code violation.
But now you tell me the downstairs lavatory discharges into a side inlet closet bend and the lavatory vent continues up through thr roof. This makes you completely legal and completely within code.
"So the lavatory 2" pipe drains into the side of the closet elbow of the toilet (and lavatory has its own vent through the roof)... then the toilet closet elbow goes directly into the main stack (which I thought is it's vent)"
And this is where we went off track. you mistakenly called the stack a vent and I took you at your word. No wonder we were at cross points.

"what you are saying is (correct me if I'm wrong) the toilet and lavatory can use the lavatory vent now and the toilet does not need to have a separate vent?. they can share the sink vent?. even though the vent that the toilet would be sharing is also being used for the lavatory drain?"
Yes, the toilet is wet vented by the lavatory vent.

Forget everything that's been said so far in the earlier posts. You may install your 2nd. Floor bathroom group and discharge it into the stack that services
Both bathrooms and snce the basement's a dry vent you may revent the group back into the basement vent. I apologize for the earlier confusion. Your basement's legal and you have a perfect setup for both drainage and venting when you install the 2nd. Floor bath. Good luck, Tom

Yes my apology for the confusion. This is very difficult for me to explain something that I really don't know the terminology... but hopefully I get it right

That last message took me a half hour of editing it because I kept confusing myself... so sorry again for the confusion

THANK YOU SO MUCH for the quick replies and everything I've learned just from reading all your comments on other posts !

BASEMENT: I have a lavatory sink and toilet that are right next to the 4" pipe/stack/vent/drain? they both drain into the cement floor. i assume they are both vented by the 4" pipe... because all I see is a sink(which drains straight to the floor after the trap), a toilet and a one 4" iron pipe ... nothing else... this is where i think the lavatory needs a vent which will then wet vent the toilet...I would like to use a studor vent because it would be the easiest for me...
so the sink drains into the toilet and the toilet uses the 4" pipe for venting... but I cannot see what happens under the cement floor because I haven't dug the cement floor up here... but HERE in the basement they go in this order in line:
Lavatory drain ------->toilet------->4" pipe
so what is assumed is that if i did put a studor vent under the lavatory sink IN THE BASEMENT (not 6" abaove this or that) my basement lavatory sink will be vented by the studor and the basement toilet is using the wet vented by the lavatory vent... RIGHT ?

FIRST FLOOR: (includes a toilet, sink, and bathtub)
The 4" pipe goes from basement floor straight to the closet elbow for the first floor toilet. this first floor toilet is wet vented by the sink AND NOW I JUST FOUND OUT THE BATHTUB DRAIN GOES INTO THIS 2" SINK DRAIN WHICH GOES INTO THE SIDE OF THE CLOSET ELBOW TOO. So it's the same scenario but I've just added a tub drain. So the entire bathroom uses this same 2" vent ... This is ok right ?

I didn't know the bathub drain went into wet/dry vent of the sink ... i didn't see it until i opened up the tiled wall.

2nd floor this is where i want to install a toilet and lavatory sink to this same 4" pipe on the second floor and I will properly vent this group.

We are now back to square one. You first told me this, " MY FULL BATHROOM UPSTAIRS USES THE HALF BATH IN THE BASEMENTS VENT and then you told me, "So the lavatory 2" pipe drains into the side of the closet elbow of the toilet (and lavatory has its own vent thru the roof)... then the toilet closet elbow goes directly into the main stack (which i thought is it's vent)" I thought you were referring to was the basement toilet and lavatory with that statement.
Now you say, "BASEMENT: I have a lavatory sink and toilet that are right next to the 4" pipe/stack/vent/drain? They both drain into the cement floor. I assume they are both vented by the 4" pipe ...because all i see is a sink(which drains straight to the floor after the trap) {This make ir a illegal "S" trap}, a toilet and a one 4" iron pipe... nothing else."
This puts us back to square one in this continuing saga.
"so what is assumed is that if I did put a studor vent under the lavatory sink IN THE BASEMENT (not 6" abaove this or that) my basement lavatory sink will be vented by the studor and the basement toilet is using the wet vented by the lavatory vent ... RIGHT ?"
Correct!
" The 2nd floor this is where i want to install a toilet and lavatory sink to this same 4" pipe on the second floor and I will properly vent this group."
As I hope you will remove the basement "S" trap and vent the basement to code.
On the 1st. floor bath you say, " I've just added a tub drain. So the entire bathroom uses this same 2" vent... This is OK right ?
This is correct.
I apologize for all the misunderstanding but I think I have your lay out pretty well understood with these last few posts. Regards, Tom