Depends how old the hedge is, but conifer roots are more of a fibrous nature compared to the monkey puzzle you asked about previously - you wouldn't try to plant anything within a metre minimum at the base of a leylandii hedge, so allow up to 2 metres either side for roots, though there may be a wider spread if the hedge has been in a long time.

Agree, tend to have a single tap root and the rest spread out below the surface. Difficult to grow anything near as they denude the soil of water and nutrients.