Should I not use double things in a double room if I have booked it as a single guest?

You posed this as an ethical question, and while you’ve gotten really good answers about circumstance, no one’s addressed the ethical aspects of the question, so I will here.

You ask if it is ethical to use the consumable items provided.

My answer would be yes, it is – you should use what you need, and no more. Don’t waste them, but don’t avoid using them out of some sense of ethicality. If you need to open that second bottle of shampoo on your first day, feel free, if you require it. But don’t waste the resources unnecessarily. (The more quickly you use them, the more quickly they need to be replenished, which unnecessarily costs the hotel money.)

The top post in this thread discusses how hotels often discard all consumables after a room is vacated – I had never contemplated this before. If this is the case, ethically, we should take consumables with us, if we’ll use them or can find a use for them, so that perfectly usable products aren’t discarded. (Maybe it’s the Scot in me via my ancestors, but I put my partially used soaps back in their wrappers, bring them home, and finish them off in my own shower. Then again, I’m the type that sticks the remainder of my old soap to the new one, so I’m not wasting any of it.)

In general you are free to use all the amenities in the room, as, for example, the linen on both beds will be changed when you check out whether you slept in one or both.

This is not universally true — I once stayed with a friend at a budget accommodation somewhere in Europe where we were told we would be charged extra if we used both beds in the room.

As for things like soap and shampoo, they are yours to use. Balding men pay the same room rate as long-haired people who need two or three bottles of shampoo to properly clean their hair. (In fact, I have never had a hotel or motel refuse my request for extra shampoo.)

(Source: personal experience. Posting to share the story of the hotel room where my friend and I were told to share the double bed or pay extra.)

Being a supervisor at a well known 3/4 star hotel I speak from experience.

If a consumable/expendable amenity has been provided in your room, it is yours. This includes personal care products, snacks, coffee, toilet paper, etc.

Housekeepers aren’t supposed to leave these items for reuse anyways and should ideally be thrown away whether you use them or not. You never know what the guest may have done to an item that appears to be untouched. I heard of a location in Texas that had a guest check in and find drugs stashed in what appeared to be a untouched shampoo bottle.

Taking home items that are NOT depleted by their use is theft. A good example would be towels. Do not take towels home. You may however use all amenities provided to whatever extent and for whatever reasonable purpose you wish. We may not like it, but you can substitute towels for toilet paper. It may be gross, but they will be fully cleaned, sterilized, and reused.

Use of provided amenities is factored into the cost of your room and I believe that you should always use them to get full value for your money

My hotel chain does not provide alcohol in the rooms unless requested with official ID, so I cannot say if alcohol is yours for the room rate when stocked prior to check in.

The number one thing that is appreciated by staff, is to leave your room as clean as possible at check out. The time to clean the room is the number one expense in a room where nothing had been stolen or damaged.

Keep in mind that most hotels will charge you for non-disposable items such as towels or fixtures you steal, and may even press charges if the value is high enough. The same goes for most damage to a room so if you enter a room and find that something is obviously missing or damaged, report it to the front desk to avoid liability.

EDIT: It is also acceptable to move furniture that has not been secured. If you want to join beds together, bring a table from another room you have paid for, or overturn the dresser you may do so. It might be odd and not appreciated, but as long as no damages are incurred that require maintenance to perform repairs, it is acceptable. Simply put, if you need a tool to do it, or undo it, don’t.

YES, feel free to use all the in-room amenities as you please.

Be aware though that all rooms are provisioned the same regardless of how many occupants are expected, single being the hard exception.

Meaning, housekeeping rarely knows how many people will eventually occupy the room so often you will find 4 person rooms also provisioned for 2.

If larger parties require more items, they will be provided without question.

Use what is necessary if it is available within the price of your booking.

If you start going that way there are many more things to worry about. Should you use half of the room and not step in the other half? No you paid for the whole room and everything that comes included within that price is morally and practically fine to use. Most of those things will get thrown to bin anyway, why let them go waste if you can use them?

They aren’t charging you half price of that room because you are single and the room is twin. So use what you paid for without feeling guilty.

The argument in comments on this answer is assuming that the customer always knows what % of the room price they are paying. That’s flawed. Customer doesn’t know and shouldn’t care what % of the cost are they paying for the bigger room. They are paying what was asked for by the hotel and that’s that. I’m paying 100% of the cost. Its none of my business if it costs the hotel more than that. They get what they ask for.

If you someday get a free (or for a nominal cost) flight seat upgrade from economy to business class should you refuse it because you didn’t pay for it as much as other people did? Nope