The Act provides the right to raise a Party Wall although it is not specific in what materials can be used.
Party Wall Surveyors prefer a wall being raised in matching materials, usually brickwork. This provides a better detail for the works and also allows a neighbour to make use of the same wall in future. It may be slightly more expensive than creating a timber/tiled dormer however, if/when the neighbour builds a loft in the future a share of the cost of the wall will become due. This cost would be calculated based on construction costs at the time of the neighbour’s works.
Raising the wall in timber/tiling is a more awkward detail which needs to be carefully designed to ensure that the neighbour’s ability to do similar works in the future is not prejudiced. There would be no cost contribution at that time and the future works have the potential to be disruptive as they would likely need to strip the some of the tiling and battens. The weatherproofing at the junction of both structures will also need careful consideration.