Hi, all I can say is I am just about to complete a new build dormer style 3,100sq ft house, timberframe, and we were way over the budget you have by 160,000ish.Saying that, we got a handmade kitchen whereas you could go ikea, we got solid oak doors,you go pine. You will save heaps getting some stuff in England (esp if you have a VAT number). We got a good deal on engineered antique oak floors off carpenters of york for €28 sqft floors over in England. You can do stuff like that for the interior, right price tiles in January and got huge discounts for floor tiles. Just got to research. We got expensive wooden windows whereas you can get coloured pvc or something. We got sandstone cills and coins, you go cement.
Saying all this though, our costs are high enough even though we had contacts which are vital in keeping costs down. We had a friend who did our foundations and another who is an electrician. I am painting the interior myself and the doors, so that is quite a saving and most other jobs, CASH IN HAND!!
I would say by managing it ourselves, we lost €15,000 down to naiveity and trust. But I couldn't get over the cost and amount of the materials, i.e. cement, sand, bricks, pipes, plaster etc to get the house up, the foundations require an awful lot as do the walls and roof of the house. Go to a local building supplier and ask him can you open up an account and pay cash in hand and you will buy everything off him. He will more than likely take that offer than lose you to somebody else. Then there is always the labour, get the price before they start! We did go for the passive solar air tight house, quite expensive but you could always go for the cheaper radiators and oil etc,. but obviously not cost effective in the long run.
The costs are really all up to what you choose to go with, good research and most importantly, who you know!