OK so it looks like this has been incorrectly tagged as a separate residential building (building=residential) and this is why StreetComplete is asking about the house number.

If it’s an extension, then it’s not a separate building at all, so the most correct way to map this, I suppose, would be to change both the original house and the extension from building= to building:part= and then draw a new outline around the building as a whole and tag that building=. But if we want to do this for all of Edinburgh it’s a ton of work! (There are a lot of extensions drawn as separate buildings)

As a temporary solution we could just change it to building=extension