All of the above but especially what cjel said. I am coming to the end of an extension and I WISH I had made more decisions before the building work started. Anything you can make decisions on in advance will help you reduce the stress and keep you on budget. GO SHOPPING - online, or for real and look at magazines for insparation. Pick out the styles you like then research cheaper options on the high street, then decide which style you are actually going for (def see it in the flesh). Remember you will need to think about where will your light fittings be, what will they be and where will the switches go? As it is an extension you'll be governed by environmental laws so research what type of environmental bulb you like best (they can give off such a different hue etc). How many plug sockets and where? That means you have to know in advance where will your TV be along with all your electrical items. Design wise you'll be inspired when you see that thing that you love and must have. I have a thread going about the purple sofa I just bought - sometimes your heart rules over your head. If you plan you can try to find that item at a good price. Good luck!
Magazines to start with. Do you like Ikea/Laura ashley. John Lewis ? Find one thing you like - Carpet, fabric sofa? build a look round one thing. Scandanavian, english country/cosy/ minimalist? I have done more mocha/Joas white this year with cream/coffee some rooms with dark red accents and bathroom with spearmint accents. Just keep looking around and when something leaps out at you follow it!! Any Help?