What is the additional cost of a half-hip roof?

We are planning a double story extension to the rear of our house. The original roof planned was a simple gable end. Neighbours have complained that we are blocking out their light, so I was thinking as a compromise I could slice off the corner of the roof ie. creating a half-hip (barn-hip) end. I'm guessing though as this looks a bit more complicated that it would add to cost... but how much by roughly? The roof covers a footprint of approx 3.5x7.5m. Thanks for your help!