We moved into a house last year which is around 35 years old. The house had been vacant for 1 year before we moved in. We didn't notice the smell at first but after a few weeks we noticed a sewage smell in the basement where there is a fairly substantial manhole cover (inside the house!)
Over the past year the smell hs got worse and can be smelt all around the house at times, through plug drains etc but mainly it's the strong smell from the basement disseminating around the house - its not constant but is frequent and doesn't seem to form any particular pattern with running water I.e. After a bath, dishwashers been on etc.
We'd really appreciate some advice before we call a plumber which no doubt will cost an absolute fortune here in Switzerland!

Are you on septic or town sewer?
I'd look under the cover - maybe a pump, and a break in a pipe or a small leak.
Check around the house to see if any drains don't have traps.
Look at your roof to see if there are enough vents for the kitchen and baths (possibly shared), and how many trees might be dropping leaves down them. Animals can fall down them too.
What are the drains in the basement made of? Have you gone all over them with a flashlight, drips etc?

If the house has set empty for the past year I would be giving a hard look at the basement floor drain. If the seal evaporated you would discharge sewer gas every time you drained or flushed. If the floor drain won't be used fill the trap with cooking oil or anti freeze and forget it. Good luck, Tom