I rather like Mr Screwpole's classification of projects.
However, I can take it personally, because I have a personal interest in building a house extension ATM. We have plenty of vision and plenty of experiencer on the teams involved and no inclination for micro-management.
All the quotes are fairly imprecise and all the workers very plausible and, if it all works we'll no doubt be very pleased that we just set things in motion and let them get on with it - and the listed building people provide a degree of 3rd party inspection to keep everyone honest.
However, there are times when I'd quite like to have a formal and detailed specification of exactly what we're getting and when. Even though I realise that this would cost more and, even then, is negotiable in the light of what actually happens when work starts.
If I was building a mission critical business system, I'd probably place an even higher value on comfort. Screwpole's manager on the "voyage of Discovery", say, has to have lots of people skills and dispense lots of comfort to all the stakeholders on the voyage - just delivering eventual suuccess may not always be enough