1930s house - front elevation rendering

[TurnUpStuffer]

Both 1930s semis have the same white rendering. Judging by other houses in the street, this is likely original. Does anyone know about the thinking behind this 1930s design? What is this white stuff made from? What is underneath it? Does it require specialist tradespeople to maintain it?

It's just conceete or limecrete mixed with sand and pebbles and then painted. It can be mostly ignored as long as it gets painted regularly. Then as it ages it will blow in parts (come away from the wall). At this point, the blown bit needs removing and patching..eventually the whole lot needs to come off and be replaced. Check when you get it patched or replaced that you stick to lime or concrete, whatever is there already. You might be able to add insulation if you get the whole lot replaced.
A regular builder should be able to re pebbledash a building.