Costa Hawkins questions - San Francisco

[Dee Smythe]

We've got a house with apt. below. Rented it 4 years ago to four students (mistake #1). Only 1 is left. We fully believe he's no longer living there, has converted (not structurally) it from a 3-bdrm to a 5-bdrm and is renting them all out making a tidy profit. Rent is paid on time & is about $500-600 below market. We want him out but proving he doesn't live there is tough. SF is so tenant-friendly, he could ask sub-tenants to corroborate that he lives there, otherwise they'd all have to move. How to go about proving this and making sure we prevail? Thanks.