House plumbing vent system, 2nd vent?

Sorry - long post

I don't do plumbing, not allowed here. So I have never bothered too much to learn the codes. Plumber says this is what needs to be done & he does it. I do know how to solder, replace/plumb PVC/CPVC etc. I have a 240g saltwater aquarium setup that has more complicated plumbing then most of my house

My house has a 3" vent stack on one side of the house
The bathrooms are located on that side of the house
Town water & sewer
With work on that side the plumber cut off the CI & PVC goes from basement up & out the roof. In the basement the main waste going out is CI as is the 1st floor toilet connection. No problems with anything the plumber did - great job

The other side of the house has the washing machine
The kitchen sink, and will have a dishwasher eventually
Kitchen sink & washer drain into a 2" pipe then into a 3" pipe that runs over to the other side of the house & connects to the CI
The washer has an AV (air release?) in between the floor joists
The kitchen sink does not have ANY vent - dual sink
There used to be 2 windows over the sink, garage on that side now, windows gone

The 2" pipe then rounds a corner & connects to 2" CI. The 2" CI runs up into the 1st floor wall & converts to 3" CI. The 3" CI converts to PVC before going thru the roof

So, the washer may be moved to the other side of the basement near the main stack/drain. My preference, need to ask the wife
The sink will be replumbed once the kitchen is renovated
A vent will be installed at that time

My main question is (after all this)
Do I need the 3" vent on the side with the kitchen sink?
If it could all be 2" (even 2.5") then it would fit inside the new garage wall & would exit the new roof "easier"
I'm just trying to figure out why they went to 3"?
Just in case it was ever needed?

vent

WE do not know the exact circumstances even with your explanation. Normally it would never "be needed", but individual circumstances and/or local requirements could change that.

A larger roof penetration may be called for by local codes, especially if you are in the northern part of the country...this is to prevent it from closing up with frost during the winter, not because it can't handle the venting the rest of the year.

Also, it used to be code in some places to have a minimum of 1 3" vent through roof.
2" would work for you but I've seen in places up north they have to upsize the vent due to freezing.

I actually have (2) 3" vents thru the roof
I think best solution is to put a 45 in & go into the attic of the new addition. Then a 2nd 45 & straight out the roof. I was hung up on installing it in the wall - which is not needed
Thanks