There is a restaurant that occupies a whole building, should I tag all the details on the building and just have that or should I leave it as a building and add a node and tag the node?
Really depends on how things are usually done by other mappers in your local area?
Personally, I always add the address etc details to the building outline, then add a node for whatever business is operating inside.
Oh, and remember, ‘one object in real life, one object on the map’. So either map the restaurant as a node OR an area, but not both.