Upvote:4
In general, you don't have to return a car "clean" - they expect they'll have to do a normal wash and vacuum job.
The point where you normally would get charged a cleaning fee is if a normal wash and clean like you'd expect from your corner car wash is not sufficient to clean the car. In your case it's not clear what that was, but probably if they had to shampoo the carpet or something more out of the ordinary. The comment example about tar on the exterior is something that would count as well - a normal wash doesn't get it off, so they have extra work to do.
In practice that's a button that gets pressed if:
They are sketchy and just looking to make extra money off someone (in which case any dirt will do to prove "it was dirty"). It happens, but rarely.
The worker prepping the car had to go out of their way (incurring extra effort and/or expense) to get it to the next patron. That's much more common.
In the end any contractual clause is going to be up for interpretation, so I wouldn't expect a very detailed description of "what's OK" - plus they have zero incentive to not have it be "it's OK if we say it's OK." So push back on charges from normal dirt, but be understanding if you did incur a problem for them. Ask, "if this were my car, would it be clean after a trip to the car wash (without paying for extra services)? If yes, then it should be fine; if no, then it's your responsibility.
Upvote:4
This heavily depends on the country and the rental agency.
Major agencies in the US care very little about dirt that is removable, and in hundreds of rentals I have never been asked to pay anything (and some of those cars went two weeks through offroad and mud...).
The cheaper and shadier a rental company gets, the higher is your chance to get milked for some extra money, and returning a car even slightly dirty is an easy reason to claim - after all, they already cleaned it when you get a chance to disagree, and you are typically already far away.
In Europe, rental car companies are generally much more picky - they try to make you pay for every little dent, scratch, and fly speck (even the ones the previous renter already paid for), and anything more than some dust can result in "extra cleaning needs". Again, the chance for pickyness (and shadyness) increases with cheaper agencies.
Either way, you'll hardly find a clear definition beyond 'more than the usual usage dirt'. If you want to avoid that they milk you, you can ask them when returning the car, take some photos on their site, and have them confirm that it is clean enough. If they say it needs extra cleaning, drive it back out and clean it yourself (even if you pay for that service, you'll probably save money). But it doesn't guarantee that they don't charge you anyway, just because they can.