While some less-than-pleasant smells are often associated with bathrooms, DIY homeowners can sometimes be faced with odours out of the ordinary.
This common occurrence is often due to the evaporation of water in the floor waste s-bend. While the bend in the floor waste is specifically designed to stop the fumes from the drainage pipes escaping, if the water is not replenished then the s-bend dries up - allowing fumes to escape.
The simple solution is to carefully pour a bucket of water down the floor waste. It is difficult to give an accurate measurement of required water, however, if the grate is removed, you will often be able to see the water sitting in the bend.
You will want to ensure full coverage of the s-bend, so there should be no hesitation in emptying the entire bucket of water down the drain. Replace the grate - if it has been removed - once the job is complete.
Hopefully at this point DIY homeowners should begin to notice that the smell is disappearing - if not, you may have to seek the professional advice of a plumber.