What could happen if we added another housemate without the landlord's permission?

We have a 3 bedroom house with 4 people (2 are sharing a bedroom) and we want to add another person, sharing a differet bedroom. We asked the landlord if this would theoretically be possible and they said no. So far, they have been very hands off landlords and I don't think they would find out if we informally sublet a room. What would happen if they did find out?