I am totally slapping myself in the head now because I never once thought about including a secret underground party lair in the plans. Now it's too late. And I suspect Shelly will be closely watching the construction phase, asking a lot of questions such as "Is that the pantry or the entrance to a secret underground party lair?"
To make things worse, I am exactly the sort of person who would build a house with a secret underground lair. But I probably wouldn't use it for parties because I'm selfish. I would just say I was on a business trip and go hide in there. All I'd need is a TV, refrigerator, and a toilet. It could all be in one room. I wouldn't even need overhead lights as long as the TV was always on. And obviously a chair would be redundant if you have a toilet. Easy peezy is the way I like it.
I'd use my secret lair to watch all the TV shows I can't watch with my family, such as Battlestar Galactica, and Southpark. The only risk is that it might sound a bit pathetic when the authorities raid my house for Garfield copyright infringement and discover that I have an underground television lair. There is no type of beard I can grow that would make that situation seem cool.