Did my own single storey extension about 3yrs ago. Did it on a building notice so didn't need to go through the pain of planning permission.

Did have the benefit of working in the trade and my old man is a brickie.

Most time and money was spent in the ground, keep an eye out for main drains running through your garden!!!!!! Started digging the foundations the day after my little boy was born, wife not impressed.

Started in feb and we were using the kitchen for Christmas and that was just working saturday Mornings and Evenings towards the end. All in just under 10k including Kitchen.

Best bit of advise: Keep the building control man on your side....

If you do go ahead drop me an email and I'll put in touch with a couple of good suppliers for your bits and pieces.

Matt