Building costs vary wildly but a very rough guide is about £1500 per square metre but there are so many factors it's a bit like how long is a pice of string!

Looking at your plan if you are trying to get the best value per square metre I think you would be better with a square or rectangular plan rather than the L shape you have at the moment. If you took the living area wall back to level with the back of the house it would reduce the complexity of the roof whilst adding only a small amount of additional exterior wall. I appreciate you are planning a small house but the kitchen looks small especially as you have a table in there, by increasing the living space you could move the table out of the kitchen. Also I would look to open the shower room door into the room rather than into the hall as it looks very tight.