to get a mortgage for a site and new build together, the site would have to have FPP and the cost of the house to be estimated either by a quanity surveyor engineer or builder. Usually you buy a site subject to planning, you pay a deposit for the site (eg 5K) to the EA and then apply for planning, once planning is approved, the sale of the site can go through, and you pay the balance of the site price, this can take anything up to 9 months, while you are waiting for planning approval, you would have all the costs to build the house sorted for the bank and they can finalise your mortgage, you then draw down the money as you require it for each stage of building. The % amt for the mortgage varies on if its site alone which is 80% of the value or if it comes with a building agreement, then its 92% (I think).
If you can get a site with FPP, you can attach a building agreement on to the sale (with the sellers approval) which will reduce your stamp duty liability..(this is legal, its like if you were buying a house in a estate development)