There's a quite a range of flat pack houses available nowadays , the triple glazed, highly insulated packs are usually constructed to passivhaus standard and are as you say expensive. There's a number of UK companies that produce flat pack houses and are available in a number of styles and sizes. As they are not of traditional construction planning committees can ve a bit sniffy about them , but they can be built in a much quicker time frame, are more heat efficient as well as being tailored to a particular use before it is even erected at the manufacturer. The more thats made the more likely the prices will come down.