The 23-year old will put pen to a deal with the Bluebirds early this week according to manager Malky Mackay who is a big fan of the former Brading Town and Farnborough man who joined the Bees in 2009.
"It'll be paperwork first thing Monday morning, that's the only thing holding that one up," Mackay told the Hounslow Chronicle.
"I'm glad to get that one in, he's someone we've tracked for a year and he's had a successful year for Brentford.
"He's one that I think is a real prospect. For the future of the club, I've got three really good goalkeepers now; Simon comes in and I think he can kick on again.
"My thanks to Brentford for the way they went about their business, they were very professional and it was done very quickly."