Stick to a proven and already used house design, it will save around 25percent compared to a oneoff design you get drawn up.  

This is where i disagree - the franchises will say its a proven and popular design, but they are looking from the point of view of ease/cheapness of construction to make the most profit.  Yes there are some OK designs put out by these sorts of mobs, but if you have a few particular things you would like in a house, many of them won't budge.  I would much prefer to pay a bit more to build something that is the most practical for my living needs.