"The bathroom is next to the restaurant."
As Diego implies in his response, your construction reverses the subject and object of the sentence.
Instead of 'The bathroom is next to the restaurant.' your construction is more like 'Beside the restaurant there is a washroom.' Your Chinese isn't wrong, but it reverses the sense of the English.
洗手间在饭馆旁边 worked fine for me. They must have added it! On a separate note, are restaurant bathrooms in China outside the "main" establishment? Of course, I am not referring to modern buildings. The expression reminds me of old gas stations in US where the bathroom(s) have separate doors from the main gas station store...
In older districts there are often public toilet buildings, usually of two storeys (some apartments in these areas may lack their own toilets). Nice restaurants always have washrooms, but small cafés/restaurants in these districts mightn't and could conceivably be next to a public facility. That's what I imagine this sentence suggests (or else the truck-stop possibility.)