Secure apartment reservations for 2018

If there is not a 13-digit registration number on the website, it is an illegal apartment. The number cannot be faked, it is tied to the property owner's financial information, and there is only enough space on the electronic form.
That's really all you need to know.

Booking.com and other third party websites might not have gotten around to posting the 13-digit registration number. But if you do not see it, I wouldn't rent the apartment.
There is also a chance that the apartments might be sold, during the all the commotion. It takes about 3 to 6 months to sell an apartment.
You wouldn't want to get to Paris and not have a bed...

As stated elsewhere many times.

An agency such as Booking.com ONLY lists apartments legal or illegal. The agency has NO responsibility regarding the rental laws. The property owner is responsible for the registration. The ONLY protection a renter has is the registration number indicating a legal apartment. If you rent an illegal apartment and it is pulled from the market, the rental agency, unless otherwise specified by the agency, has no obligation to you.

I still don't know the answer to this and I hope someone on here can answer. Are the registration numbers required only for Parisians who are renting their primary residences for less than 120 days? Are apartments that have, what I will call for lack of better words, commercial licenses supposed to have that registration number? What I am calling a commercially licensed apartment are the apartments that have been registered with the Mairie of each arrondissement, the ones found at https://opendata.paris.fr/explore/dataset/registre-des-autorisations-de-changement-dusage-pour-les-meubles/information/ or first floor apartments. I know someone who has been renting the same first floor apartment for years for vacation and the owner says that the rental is legal and his apartment is a commercial space and will not need or get the 13-digit registration number.

" I know someone who has been renting the same first floor apartment for years for vacation and the owner says that the rental is legal and his apartment is a commercial space and will not need or get the 13-digit registration number."

Clarification necessary. In France the first floor (American) is the ground floor.

You can safely assume the owner of the above ground floor apt. is correct. . In general most NOT ALL ground floor apartments (American first floor) were exempted under special cases, or grandfathered, or had building approval or previous commercial usage. As far as I know they do not have to be registered. However, there must be some type of commercial licensing for these apartments but who knows where to look. Ground floor dwellings have different status than those of persons wishing to get a commercial license for their secondary residences enabling them to rent them short term. The fees are incredible and there is a clause stating that a second apartment must be purchased to be returned to the long term rental market in order for the short term apartment to be rented short term.

As for the fine points, maybe you have a lawyer friend in Paris.

I meant ground floor. Thanks for the response. I'll skip getting a lawyer in Paris because this is just curiosity on my part. The thing is though is that if there are some apartments that do not need a registration number, where can a renter find out if those apartments are legally rented. Is the only place to check at the link that I posted? I find this system very confusing for people trying to rent legal apartment and am curious to see how it all plays out but this is the curiosity of an observer and not of a person who is looking for a vacation rental so I can watch and see what happens without worrying about my accommodations.

The co-propriete as a whole must decide whether or not the ground floor apartment (rez-de-chaussee) is commercial. The tax code will have to be changed, which residents don't want to deal with.
It could be that the gardienne has been removed, and an enterprise takes care of the building, so it could be voted as a commercial property.

Any agency - AirBnB, booking.com and VRBO/HomeAway and others are listing properties, and are not breaking any laws.
The problem is if your apartment has been fined, or the property owner has plans to sell the apartment, you will not find yourself in a bed.

The property owner must apply for the registration number. If the property owner has a tenant, it must be specified in writing that he has the right to sub-let. This is usually not permitted by the tenant's lease, even if it is a ground-floor apartment.
Look for the 13-digit registration number and you will be all right.

I wonder if the sites that control an entire building as rentals would have to have the 13 digit registration. Two companies I know of have buildings with nothing but apartments or suites as rentals.

If the property owner does not actually live in the building full-time, he cannot apply for a 13-digit registration number.

If the property owner wants to list his property as a commercial venture - say, a hotel - he will have to go through the City's process of safety codes, plumbing and wiring codes, and access for handicapped people, to mention a few. He or his company will also have to pay a commercial tax and charge a room tax for his guests.
Then, it would be a hotel.

One company on Place Dauphine seems to have a time-share going for it's wealthy patrons and a few of their guests.
There is another owner who claims that the whole building has been "in the family for generations" and is exempt from the registration number. This is not true.