is this doable?
Just putting a few feelers out really to see if this is acheaviable on the budget I have as no idea on material costings etc.
Any advice?
It depends on your attitude to work more than anything. You won't get it in at 16k by surfing eBay and drawing project plans in Excel, but actually doing most of the work yourself might. I DIY'd a 5M by 3M two storey extension with a cut roof and it came in at about your budget, so it's definitely possible, and there's enough knowledge in this forum to get you through most things. As Noseall says, you have to take your time sourcing and be prepared to inconvenience yourself to get the best prices, and hope that you have a very understanding other half as you make her live in wet/dusty/cold hell for 18 months. Bear in mind that it's a lot of effort and it's not really a 16k extension, it's just that you're not counting the 25 hours a week you'll put in to it over 18 months I needed help with 4 man days bricklaying, 2 man days plastering, around 2 man days electrical, and about 10 man days advice from this site!