venting for additional bathroom

My problem is how i should vent an additional bathroom that i would like to add to my 1930 house. It currently has only 1 bathroom, which is on the 2nd floor. There is a 4" drainpipe and a 3" vent, both of which extend from the basement to the 2nd floor (and the vent thru the roof). It's impractical for me to add a new bathroom either near the existing DWV lines, so i would have to convert a mud room in the corner of the first floor house into the proposed bathroom. I can easily add a drain from the mud room into the basement, but i don't know where i can add a vent pipe. Adding a bathroom would be impractical if i have to tear the house up to add new vent pipes.
Here are the options that i can think of so far: 1)Is it acceptable to have a vent pipe that extends upward @6" from the trap, then back down into the basement to connect to the vent pipe there? Or do i have to have the vent pipe travel upwards and over to connect to the vent pipe above the highest trap in the house?
2) can i get away with a "cheater valve" which means i wouldn't have to install new vent pipes?
3) do i need a separate vent pipe at all? if the waste line is 4", might there always be air in the line to serve as a vent?
I live in Massachusetts, and i'm having a hard time interpreting the state plumbing code. I don't plan on doing the work myself, but i need to know if this is even feasible, and i want to know if i should be suspicious of a plumber who proposes the most expensive solution.
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davidmc wrote:

In Mass., count on the approved solution being the most expensive.
I can't imagine that they would allow a mech vent to serve an entire bath. Nor a vent which loops downward.
Take a deep breath and have a man-to-man with the town inspector. He may alert you to other requirements you haven't even thought of yet.
Jim
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Jim makes a good point talking to the town inspector. As well as letting you know what other requirements you need to be aware of, they will likely give you options that they are willing to approve.
I find that being a homeowner and taxpayer, they are more that willing to work with me. Contractors on the other hand are getting paid to do a job and are supposed to know, so they are less helpful with them.
David
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Your questions deal only with the toilet. Bathrooms usually have sinks and tubs/showers in them. They will need vents as well.
Vents go up, not down. Your description makes an inverted "P" trap. ( at least in my mind )
Call some pros and find out what is needed for a proper installation. Getting 5 or so bids should make it easy to understand what a proper installation will entail.
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after a little more research, i found out about something called a "bow vent" which can be used for drain/venting a kitchen island, which is comparable to what i want to do. would this work in my situation?
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davidmc wrote:

It may and you will still need to ask the inspector first. Jim
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Well, mostly, but they do do that sort of thing for island sinks. I do not know for sure, but I suspect that the upward loop of the vent pipe has to go higher than the flood levels of all of the fixtures is serves.
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