When we moved in the smell in the upstairs bathroom was pretty bad. I think that they must have masked the smell when we were looking at the house because I think that it would have been a down point since you can smell it in the bedroom down the hall.
Things we tried:
- Cleaning the bathroom
- Dumping a bunch of water down the drain to make sure the P-trap has water in it
- Looking for hidden mold or dirtiness with a black light
Right now we got it into a state of acceptable with following various advice. Unfortunately the advice that worked the best got mixed in so we pretty much feel like it was a combination of various things that helped us git rid of the smelly bathroom orders.
If you are technical like my husband you could check the pipes yourself, but his uncle is a plumber so this wasn't a big deal for us. Before bringing someone out you could try unscrewing the pipe and making sure threads are clean and then put them back together making sure they are tight and well sealed. Do you have basement or anything you could see where the other side of the pipe goes that is in the wall?
The P trap may not hold water sufficiently to block gases.
This happens in little used bathrooms or those that were not hooked up correctly we think that the latter was the case since it gets used every day.
Some pedestal sinks are built hollow and the overflow drain does allow gunk to accumulate, but even though I had one of those, it never got smelly.
Have you checked the wax seal on the toilet?
Did you have your home inspected? Were there any notes on this matter? You may have legal recourse depending on the terms of your purchase if you end up needing to pay for a plumber.