Unless your friend is an experienced and qualified extension builder, I would politely decline. Personal experience means that I used a series of amateurs to help me build a small house extension. Within three years, it had collapsed more or less, and the house insurance would pay out as we did not use regulated, qualified, and insured professionals.
Learn from my mistake, please. Focus on hiring extension builders in Sunbury in your local area that you know can do the job that you have asked for. You should always ensure they are experienced and qualified and that they come with a protected insurance policy. I would like to make sure that others do not do what I did and use a cheap extension solution. In my, and others, experience? This rarely, if ever, works out as you hope.