Second those saying this will cost over $50k. Keep in mind you will need to redo much of your framing, Sheetrock, etc after you dig. We dug ours down and finished it (completely unfinished before), including adding a full bath about 8 years ago. It cost about $100k then.

This is what people in London do. It makes sense, since you can't build out!

https://www.independent.co.uk/news/london-super-rich-basement-extensions-a8415731.html

OP here: I really appreciate the input. I wish it were a little more unanimous but everyone’s points are well taken.

Probably worth it, but it is a huge, messy project and will absolutely cost more than $50K. Probably double, and then you'll want to invest in a robust water mitigation system before you re-finish the basement.