I had my house built 2yrs ago, and am now starting a basement finish.

I had 2 bathroom rough-in's put in when the house was built, both set up for bathtubs.

Bathroom #1) It seems fairly straightforward, with 3 drain pipes in place. Is there a way to figure out where a vent should be? Should it be in the ceiling somewhere? Or do I connect somehow to an existing vent in the basement ceiling?
In a correct basement rough-in (when planning for a future bathroom), is there a general rule of thumb for the venting, as to where it should be located?

Bathroom #2) It was setup for a bathtub, but I would like to make this a walk-in shower. I would like to use a preformed plastic bottom, and then tile the walls. From reading many of the questions on this forum, it shoulds like I might have to move the drain, since it was originally set up for a bathtub. Is this true? I could just install a standard bath/shower combo too...
I am trying to figure out my options.

Thanks, Dave.

"Bathroom #1) It seems fairly straightforward, with 3 drain pipes in place. Is there a way to figure out where a vent should be? Should it be in the ceiling somewhere? Or do I connect somehow to an existing vent in the basement ceiling?
In a correct basement rough-in (when planning for a future bathroom), is there a general rule of thumb for the venting, as to where it should be located?"

The pipe in the wall picks up the lavatory with a sanitary tee. The bathroom group vent will take off from the top of this tee and run out the roof or revent back to a existing vent in the attic. You may not connect this vent to anything but a existing dry vent.

"Bathroom #2) It was setup for a bathtub, but I would like to make this a walk-in shower. I would like to use a preformed plastic bottom, and then tile the walls. From reading many of the questions on this forum, it shoulds like I might have to move the drain, since it was originally set up for a bathtub. Is this true?"

Yes, First determine what base you wish to install and then move the tub trap and raiser to accommodate it. After you get the trap moved you may fill the tub dap out with cemant and cement over the shower trap leaving the raiser to pick up your shower..

Let me show you a typical basement rough in.
Toilet connects to sewer main. The toilet wet vents through the lavatory vent. Lavatory connects to toilet drain and runs a vent out the roof or revents back into a dry vent in the attic off the top the stubout tee. The shower connects to the lavatory drain and is wet vented by it. This is a normal rough in and is acceptable both by most local and state codes but also The Standard Plumbing Code Book.
Good luck, Tom

Tom: The pipe in the wall picks up the lavatory with a sanitary tee. The bathroom group vent will take off from the top of this tee and run out the roof or revent back to a existing vent in the attic. You may not connect this vent to anything but a existing dry vent.

Dave: OK. The dryventing seems like a challenge when finishing a basement. If one was thinking ahead, then dryvents would already be routed?

I have a one story house, with an unfinished attic and basement. I can get into the attic fairly easily... In my basement, I can also see all my bathtub and shower drains...

Here are some thoughts on what I might do for dry venting...

Dry vent Option 1) In my basement, I do have underneath access to my main floor's bathtub and shower drains... they are reasonably close to my basement bathrooms. Will these main floor drains (bathtubs and showers) have immediate dry vents coming off them?

Dry vent option 2) to get the dry vent to the attic (onceI get pipes to the attic, it seems "easy" from there), I might pipe through an interior wall in the main floor? This would require some careful measuring and cutting...

Dry vent option 3) The other option is to pick up an existing bathrooms dry vent behind the lavatory cabinetry... I would need to cut out some drywall and the back of the sink's vanity...

Dave.

"dry vent Option 1) In my basement, I do have underneath access to my main floor's bathtub and shower drains....they are reasonably close to my basement bathrooms. Will these main floor drains (bathtubs and showers) have immediate dry vents coming off them?"

No! As you can see by my description of a typical rough in the tub and shower's wet vented.

"dry vent option 2) to get the dry vent to the attic (onceI get pipes to the attic, it seems "easy" from there), I might pipe through an interior wall in the main floor? This would require some careful measuring and cutting"

Whatever it takes, Although I would like to see a 2" vent If things are btight and local codes allow you may also reduce to 1 1/2".

"Dry vent option 3) The other option is to pick up an existing bathrooms dry vent behind the lavatory cabinetry...I would need to cut out some drywall and the back of the sink's vanity ".

You may revent at any time back to a fixture vent if you connect at least 6 inches above the flood rim of said fixture.

"when finishing a basement. If one was thinking ahead, then dryvents would already be routed?"

You would think so but unless the contract calls for it we leave the vent to be installed if, and when, the owner decides to finish off the basement.

If things are really tight and local codes allow consider using a AAV,(air admittance vent, (see image).
Good luck, Tom