Hello Sunir, I would not call a room in a single family house an apartment. In an apartment people assume that the space is entirely theirs. A room in a single bedroom house will have shared spaces so you should market it as a room in a single bedroom house or as a coliving residence or something like that.
The reason you can't call it a one bedroom apartment is because of what we as a society have decided 'apartment' means. It means a single unit (often) among other units that are individually rented. What you describe would be a room for rent or a shared home arrangement. Why is it called that? Because it conveys what someone is renting...it's not right or wrong, just that we all agree words have definitions and we use those definitions to determine what they are describing.
@Sunir Manandhar congratulations on entering the world of real estate investing! In my opinion it all depends on what is provided in the room? If you are just offering the room that is okay but I would phrase it like " Beautiful Room to Rent in Quite neighborhood Near City Attractions". Obviously only say that if all those things are true, but something along those lines. If you are renting a room that has a fridge, hot pan, bathroom and a separate entrance then you would say " Beautiful Suite in Quite neighborhood Near City Attractions". You could call it an apartment unit at that point but, it maybe miss leading and you don't want to start off that way with potential renters. Either way there is nothing wrong with saying you are renting a room. A lot of people are looking for rooms to rent especially depending on your area. Wishing you the best!
The others are correct. An apartment will have it's own living/sleeping space, bathroom, and kitchen. It's typically located in a structure with other apartments.