A sewage smell in a bathroom is usually caused by something close by, rather than a problem with the main drain. Work through these in order.
1. A dried-out trap
Every basin, bath, shower and toilet has a trap: a bend that holds water to stop drain smells coming back up. If a fitting is not used for a while, the water evaporates and the smell gets in.
Fix: run the tap or shower for a minute, or flush the toilet. Spare bathrooms and en-suites in guest rooms are the usual culprits.
2. Hair and soap in the shower or basin waste
Build-up in the waste and trap can smell, especially in warm weather.
Fix: remove and clean the waste cover and trap. See how to unblock a shower drain.
3. A loose or failed toilet seal
The seal where the toilet pan connects to the soil pipe can fail, letting smells out around the base of the toilet. You may also notice water marks on the floor after flushing.
Fix: the pan connector needs reseating or replacing.
4. Traps being emptied by air movement
If a trap empties when another fitting drains (sometimes with a gurgle), smells can get into the room. This can be caused by a partial blockage or a venting problem. See toilet bubbling when the bath drains.
5. A faulty air admittance valve
Some homes have an air admittance valve on the soil pipe, often hidden in boxing or the loft. These let air into the system but should close to keep smells in. If one sticks open, the bathroom can smell.
Fix: the valve needs replacing.
6. A cracked pipe or poor joint
Leaking pipework inside walls or under floors can cause smells and sometimes damp. This is less common, but worth checking if the obvious causes are ruled out.
7. A drain problem outside
If the smell is in several rooms, or outside as well, the cause may be a blocked or damaged drain. See what causes bad smells from drains.
When to get help
If the smell persists after you have run water through every fitting and cleaned the wastes, or you notice slow drainage, gurgling or damp, get the drains checked. See drain cleaning.