Do you mean the shrubby Lavatera, which has a woody base? There are other Lavatera which aren't woody. If you do mean the shrub type, don't cut back now, leave it alone until March or April (depending on whether its still very cold) then cut it back hard, back to the wood. They're fast growing and you'll still end up with an enormous shrub by next autumn. Generally hardy, though may give up the ghost in a severe winter.