VAT claim on barn conversion

I have been asked by a client if I could sort out their VAT claim on their barn conversion for them. This is something that I've never done before, so I've told them that I'll have to look into it before letting them know if I'll do it for them or not.

Before reading up on it I was wondering if anyone on here can tell me how straight forward these claims tend to be and if there is anything I should watch out for. I don't want to land myself with a pain in the neck job for what I'm guessing might only be a couple of hundred pound job!  

Thanks

Dave