Tag for apartment hotel?

I would not use amenity, I think it belongs under "tourism".

In case you want wider adaptation for your "new tag", you should document it on the wiki. Otherwise no one will know about it. You should also get in touch with the creator of e.g. http://unterkunftskarte.de/ which displays all kind of places to stay during the holidays, this might speed up adaptation.

Furthermore, did you search taginfo to find out what others used ?

I would not invent a completely new tag for apartment hotels. Instead introduce a subkey hotel=* for specifying the type of hotel. According to taginfo there is already one usage for hotel=apartement, one for hotel=apartment_hotel and one for hotel=aparthotel.

Note that introducing (sub)tags should be discussed with a wider community, i.e. the tagging mailing list and/or forum.

It is important to distinguish between hotels which have apartments, so-called "suite hotels" and apartment hotels, which are actually holiday apartments with a hotel-style booking system. In practice apartment hotels have far more in common with regular holiday apartments than with hotels (I have used both extensively whilst working on long-term business assignments).

I therefore agree with the advice in the other responses:

• Use the tourism key
• make use of an existing value add use a subkey

I would, however, suggest that tourism=apartment is probably more appropriate than tourism=hotel. The choice of tag will really depend on the range of facilities offered outside individual apartments (e.g., bars, restaurants, leisure facilities etc.)