have you noticed the same responses come in from the same people?
Do you want high end, low end, somewhere in the middle, absolutely the cheapest builder???
if you want to be in control, of design cost and time, then use an architect like
www.atelierdb.com who have done lots of whole house refurbishments and extensions and can recommend suitable contractors.
in seriousness: don't put all your eggs in one basket: use a team of professionals to advise you on your needs: any cost outlay is more than recovered by:
- ensuring you make the most of your space: take time to design it (you will have to live with it for quite a while)!
- going to tender: call me crazy - but take your desires in drawn/spec format out to 3 or 4 builders to make sure you are getting the best solution for your brief.
- mitigating cost escalation from unknowns on site (ie all scheduled out at tender stage)
etc