Where's the catch?

Still slow progress in the bathroom, but tonight builder has left a message on my answerphone along these lines: your bathroom is going to look so good [when it's finished] that we really think your living room should be decorated too [because it's currently a filthy dingy tip] and so, get this, we'll do it for nothing, for nothing, materials and labour inc, before Christmas, all you have to do is clear the room and put up with a bit more disruption. I can see one catch - there's clearly going to be some plastering needed, maybe £500 worth - but apart from that I just don't get it. The living room is a filthy dingy tip, true, but how long has builder been a charity? I'm delaying phoning him back, while I try to figure this out.