It sounds like such a great idea. A conservatory gives you extra living space, a window on the world when it is too darn cold to sit out and a snug winter home for your geraniums. Not bad for the price of a family car. More than a quarter of a million are built each year, and this figure is on the increase. You can buy one at DIY stores as easily as a bag of compost and, unless you're in a conservation, heritage or other environmentally sensitive area, you rarely need planning permission to have one built.

But research at Cranfield University has found that, in the British climate, a conservatory is unlikely to provide more than two hours of comfortable temperature per day – the rest of the time it will be either too hot or too cold. If you want to use it as a permanent habitable space, then you will have to pay for a lot of energy to artificially cool it or heat it.  

The only realistic way to keep a conservatory cool in summer is by serious through-ventilation – meaning permanent vents at low and high level. And in winter there is no real prospect of keeping a conservatory warm enough for habitation without costly artificial heating; the problem with that is that all that lovely warmth will escape through all that lovely glass! 

The alternative is a sunroom or garden room with glazed walls and a solid insulated roof. The downside is that this sort of extension does require Building Control approval, but maybe that's a small price to pay for a comfortable, habitable room, rather than one that you can only use for two hours per day.