I agree with doubling what you thought.

You never know what problems are underground.. I had a storm drain that ran exactly where the footings were going for the side extension. I had to have pad stones and rsj's put in underground etc, and building inspector wanted footings approx 7ft deep. It put another 7K on the build straight away.

Best get some drawings done and get some quotes.

Oh yeah just for info.. My extension which went out 2 Meters at side, then ran length of house which is about 20 Metres then went round the back about 3 Metres and across by 8 Metres came to about 37K. That included a cheapish kitchen and bathroom.