Do you have to wash a rental car before returning it?

You have to return it in the same condition it was in when you took possession. Rental cars are quite clean when you rent them. If you're out driving on dusty or muddy roads, the $10 you pay at a drive in car wash is a lot cheaper than the cleaning fee the rental place will charge you. You also make sure that you do a thorough inspection with the attendant, both when you pick it up and return it, make sure anything you see out of the ordinary is WRITTEN onto the paperwork before you take the vehicle, and get a copy. If you don't, you can get nicked for something that was there before you had the car. And when you turn it in, get it in WRITING on the paperwork that there are no issues.

Not if it’s in a condition expected for the  time you’ve had it. They just stick it through a car wash.

They are far more concerned about the interior ad cleaning it is far more time consuming.

Food wrappers and fresh or sticky finger prints might get you s surcharge. If someone is sick in the car expect to pay big time.,

Smoking will land you with a £250 bill.

That's the rental agency's job

No …..

Nope. That's their job. Unless you went off roading in it or something.

No...leave it dirty so that the dirt covers any scratches.

No. You SHOULD clean your trash out of the inside of the car before turning it in, just out of common courtesy, but you don't have to wash it.

Check the paperwork that came with the rental. Different companies may have different rules. If you did get it really dirty then yeah, just wash it as a nice gesture.

nope they'll take care of the cleaning. all you need to do is fill it up with gas per your contract

Not normally. Unless you have coated it with mud off road of course.