Permitting a basement.

Also if you go to sell eventually, it becomes a quick and easy way for the buyer to negotiate down, because the city won't have that square footage accounted for as finished space. You make it easy for a buyer to say "not only can't you list this as finished space, but now you have to disclose it that you know it wasn't permitted." Strong negotiating point just to give away like that. I bought a house a few years ago and got a ton of money knocked off the sell price, because an addition didn't have a permit. And the guy buying my old house tried pulling that on me, but prior to listing I went and got after the fact permits done on the work the prior owner did. I got full price for the house I sold, and tens of thousands off the house I bought.