Whether or not you need to apply for planning permission will depend on whether your replacement structure falls within the constraints of permitted development. Apart from size, height and location restrictions, permitted development criteria include that materials should be of similar appearance to the existing house. Provided the main house has a slate roof, then slate would not be a problem. If tiled, then the replacement structure would need planning permission, regardless of whether it met other permitted development criteria. If you think the works might be permitted development, apply for a certificate of lawfulness from the council to confirm this.

Mike Dade (Build It Expert)