How do you tell your flatmate that you like him a lot?

"I like you a lot, actually more than just friend kinda like" is one way. You have to be careful though since you are living together and don't want things to get awkward if he/she don't feel the same. If you notice he/she does not feel the same you can always add "But I understand if you don't feel the same, I just wanted you to know".