so i have a new set of wheels rear is 9 speed shimano, thing is i'm going to build up my new frameset with shimano 11 speed ,will the 11 speed cassette fit on my existing wheel or am i buying a new rear wheel .

thanks,
anto.

no, unless it's a mavic. otherwise, new wheel or new cassette body (freehub)

Hard to say. If your 9-speed cassette needs a spacer to fit properly on your wheel, then the wheel may have an 11-speed freehub. If no spacer, it is a 10-speed freehub.

Thanks lads is there a difference in the lenght of the freehub .might be as cheap to buy a new back wheel .

Yes, the width of the cassette / freehub is wider on 11 speed shimano. Sell your current wheel and buy a new one if you're determined to use 11 spd.