'At' can be for any location, whether it's a building or not. For example 'at the bus stop' . 'At' also often refers to the function of a location rather than the physicality of being inside a building. Think of 'at school' or 'at work' - these uses of 'at' indicate that the person in question is studying or working. The focus is on why they are there. So, if you say 'He's at the pub', this could mean that he is inside the building, or it could equally well mean that he's outside sitting at a table in the pub garden, or on the balcony, terrace, or even in the car park. It's a far more general location. We know that the person is probably relaxing and drinking with his friends at particular location, but we don't know whether he is indoors or outdoors. If you say 'He's in the pub', this means that the person is literally inside the building. So you might say 'Fortunately, I was in the pub when the thunderstorm struck'. You'd use 'in' here because you want to indicate that you were physically indoors. The same goes for 'house'. A party in your house is indoors. A party at your house could include your garden/yard/patio etc. In your example, we say 'at my house' because you are referring to the function of your house - it's your home and you are offering your hospitality there. The reference to the rooms, by contrast, refers to the architecture of the house and the fact that the rooms are physically located inside the building. I hope that all makes sense.

We use "in" for fixed locations, usually with some kind of limits (for example, the total area of your house). "At" is for points in a journey through space. If you "go to" a place, then you "are at" that place. Your example suggests that the listener needs to go to your house. Look at the examples above and you'll see how "at" is a point in a journey.

I have the same answer :) "at" can mean that you are near or in a place, while in means that you are definitely inside a building. If you say "meet me at the coffee shop", you can also meet in front of it :). Also, "at" can be used for locations that are not buildings, like "meet me at the drinking fountain".

Good question. Can't be about scale (the scale of a house within a country is on a greater scale than that of a room within a house) because it goes back to "in" when you get to the global scale (in England). It might just be one of those odd conventions that defies reason. But you say "at an address" ("at 221B Baker Street") and "at the railway station" and so on. So there is consistency around using "at" to talk about a building (premises). If you were standing outside the premises, or in the garden, or on the roof, you'd still be "at" the house and "at" the address. Those relationships don't apply to a room: you're either in it or you're not ("I'm in the bedroom" or "No, I'm not in the kitchen!". Similarly, the room itself is "in" the house probably for the same reason. Ok, maybe that's the reason. :)

Katherine! Здесь причина в глаголе to stay .stay at (some place) остановиться пожить в каком-либо месте, погостить у кого-либо I stayed at my friend’s for a week.