"My mother wants a house on the coast."
Translation:Min mor vil gerne have et hus ved kysten.
Adding "gerne" is about politeness, not formality and i would always add it even when talking to a close friend if i wanted something from him. But let us say we start talking about our future plans then i might say "jeg vil have en porsche=i want a porsche". Here it would be okay to leave out "gerne" because i clearly don't expect him to give me a car.
Ah I see, so it's more like the equivalent courtesy of "Yes please" / "No thank you" and "could I have / may I have". Except less flexible and more of a requirement unless you want to be seen as rude. My only reference was Mandarin where politeness and formality go hand and hand. Thank you for responding to people's questions. (: