Summer, winter, Spring, it doesn’t really matter! Plan the construction phase around when it would work best for you, taking into consideration school holidays, temporary accommodation and available finance. British weather is unpredictable, and you can’t plan anything based on it, if it rains, you may lose a couple of days building but this is to be expected whether you plan for summer or winter. One tip I would give is you may get a better deal from your builder in winter, they don’t tend to be as busy.