If they are a couple of friends (or say two brothers) and one plans to sleep on the couch in the livingroom I think you can say no thanks.Just some thoughts.

Welcome to BP and also mic drop^

I was thinking about dividing the space in half and adding an extra bedroom in the basement, and leaving the other half as "basement space" (couch, tv, hangout, etc).

You know what...I went to plenty of parties, drank plenty of beer, had plenty of fun, while couch-surfing and commuting to school while living at home.

Often called “mic’s”.

More people sitting on couches. 

Depending on where you live in the city, you can get compensated pretty well, I've seen listings as minimal as 'sleep on my couch' so there's definitely room to get in on that.Another idea:Try and meet some investors who are looking to buy. 

If a neighbor is loud they have to get their arse off the couch and go talk to them.

This could be a room with a pool table, some couches and chairs, a big-screen TV for watching movies or sporting events, etc.Another (better) option is to combine two rooms and create one small apartment.

This has motivated me to stop setting on the couch and saying "I'm going to invest one day."