On the progress side (or lack of it today) I've had quite a struggle getting the gradient shading of the map file correct so that the hollows around the dormer look filled with snow, and everything the right size and shape to be the same as the original but with a layer of snow on it. Not sure how all this is going to work when I start combining them, but we'll see in the morning now.

Well, I had a go with combining what I did with the original tiled house. It's not great - mainly because the map files are quite different and the combination was a crude mask that erased some of the snowy version and its map file to reveal the original house underneath it. The map file for the tiled version is very steep. The map file for the snowy version is very shallow - so that the snow doesn't bleach out to being completely white all over. There are a few strange patches you can see on the darker parts of the roof to the south where the resulted blended map file is neither one thing or another.

Let me know what you think.

@JulianDracos - I live in an area with plenty of thatched buildings, and since thatch is one of the greenest roofing materials available that isn't likely to change very much. It is also about 10 times more insulating than tiles unless you have excellent insulation in the attic space of a tiled house. So the snow does tend to stick and stay much better on thatched cottages than it does on tiled houses. It's therefore not inconceivable that you would get some wind erosion of the snow on tiled rooftops and/or melt off. The tiles are warmer because they don't keep the heat in as well as thatch does.

The other question I have is: does the snow melt into tidy sharp-edged patches, or is there a blend like I've created above where the snow shades out gradually into a melted patch?

Do you mean more like this?

In this example there are two circles. Both are affected by the Edge Fade, Inner, which is as wide as the radius of the outer circle minus the radius of the inner circle. The inner circle is the knockout colour of the Color Key, which subtracts the inner circle, leaving a ring. Then the Blur fades the inner extent. The blend mode is just to further reduce the transparency and make it more part of the background than something hovering in front of it. In this example I used Screen and left transparency at 100%

I'm not sure how smooth the result will be when rendered out. This is my image at 3000 px square, but if you export too large you may end up with rendering issues.

Even with screen shots it's pretty difficult to work out what is happening here.

Maybe this tutorial by Dogtag will help?

https://forum.profantasy.com/discussion/6723

Here's another try :)

Click the image to see it larger.