Okay, it's late but will try to fill in all the details to get a proper answer.
My father in law has installed his own bathroom in the basement of his 2 year old bunglow. We tied into the existing 4" PVC drain below concrete to install sink, shower and toilet. above this addition is master bedroom bathroom with toilet, sink. shower and tub as well as clothes washer drain. there is an additional bathroom that all drains into the main drain down stream on our new bathroom if that matters. We did not install any venting, which may be the problem. our tap into the main drain in order from the downstream is,
toilet with 45 from flange into 1-2' straight pipe connecting to horizontal main drain at a 90 T fitting, then horizontal drain goes 90 upward to main floor, just past 90 in horizontal we have shower, 2" pipe connecting at 45 fitting with 3' horizontal and p trap, then above that sink 2" runs around walls but connects with 45 to main above shower. again no additional venting. to the best of my knowledge from what i could see in unfinished ceiling this side of the house, ie master bath has no venting, the guest bath on main floor down stream of our addition does appear to be vented.

now to the problems,
1- when the master toilet flushes, the basement toilet lets out a gas bubble in the bowl? My guess is we need to vent this area some how but the question is where in the line is best?

2- when the washer discharges the toilet bowl fills up and over flows with suds. the shower had just a small amount in the pipe but also had no water in the p trap yet.

our plan before calling it a day is to rearrange the washer drainage to enter the main drain far down stream of the new basement bath to avoid bubble problems the second plan was to install a vent near the base of the vertical stack or can we install it on the sink 2" drain pipe to avoid the gas pop in the toilet, as that would be easiest solution.

I am heading to bed as is midnight my time but will check this post Sunday for any advice. Trying to stay on the good books with the in laws which shouldn't be hard as "uncle dave" was the plumbing expert not me.

Cheers chris

Good morning Chris,

You installed a entire unvented bathroom group using "S" traps and then discharged another unvented bathroom group past it? Not cool!! And certainly not code. Discharging major fixtures past unvented fixtures can create a suction on the trap seals siphoning out the liquid to the point of allowing sewer gas to escape. Sewer is dangerous to your families health and the methane content makes it explosive. Your father in law sounds like he has just enough plumbing skills to make him dangerous to his and his families health. You guys need to get a licensed plumber in to straighten out the code violations in this house. NOW! Not next week!

1- when the master toilet flushes, the basement toilet lets out a gas bubble in the bowl? My guess is we need to vent this area some how but the question is where in the line is best?

"Bubbles are not caused by vent problems, vent problem cause "gurgles" caused by the fixture attempting to vent. Bubbles are caused by a partial blockage in the line, which, may in turn, be caused by a reduced flow velocity due to faulty venting in the system. To explain what happens is that the discharge from a toilet or washer hits the partial blockage and bounces back sending a bubble of sewer gas back up the line.

2- when the washer discharges the toilet bowl fills up and over flows with suds. The shower had just a small amount in the pipe but also had no water in the p trap yet.

Explained above. You need to snake out and clear the drainage lines that are affected. No water in a trap means that you have left a direct connection open for sewer gas to enter your house.

Chris, plumbing codes were written and enforced to protect the nations health. This house has so many violations that I wouldn't spend a single night in it. I sincerely hope that no children live in in such a dangerous environment and you and your father in law take my warnings seriously.
Please keep me informed. Regards, Tom

Certainly you have seen it all, and many thanks for your response

To clarify the trap for the shower had no water because it was capped, until the shower is installed, which is on hold until the aforementioned problems are resolved. The problems have lessened, according to my father in law?? since I heard the original tale. No more overflowing bubbles in the basement toilet, and only a shaking of the bowl water when the master toilet flushes. The current solution proposed by my father in law, i.e. correct me as able because he is strongly against the plumber and currently considering consulting books which I highly doubt address our situation, is to reroute the washer drain across the basement to the second stack. This will eliminate the suds and likely cause of increased gas build up. Then the second part of the plan is to vent the bottom bathroom sink back up to the orignal opening for the washer. This would in his words allow the air pressure to cirulate in the system? Is this any where near standard practice? Or is he shovelling from a bad pile. I do have a considerable sway with him as far as what to recommend but obviously don't have a clue with the plumbing portion.

Again any advice is appreciated and at the end of the day the certified plumber is the easy but expensive solution that I know he wants to avoid.

Thanks for the assistance, this is a great and priceless resource.

Cheers chris

I can only repete that EVERY FIXTURE that has a trap MUST be vented.
Fixtures that are not are subject to trap seal depletion and sewer gas leakage.
You say,"Then the second part of the plan is to vent the bottom bathroom sink back up to the orignal opening for the washer. this would in his words allow the air pressure to cirulate in the system? is this any where near standard practice? or is he shovelling from a bad pile.?"
He's planing on venting the basement bathroom group back to a washer "drain"? " is this any where near standard practice?"
Standard practice? Not in this world it isn't. A drain is a direct connection to the sewer and we don't vent to sewers. Your father in law isn't building a basement bathroom group, he's building a plumbing disaster. See if you can talk him into this. Call a licensed plumber out for a service call charge and pick his brains. He can show you what's wrong and how he would correct it and all for the price of a service call. You guys need qualified help bad. This is the least expensive way I can think of to get it to you. Please keep me in the loop on this. Good luck to both of you. Tom