Sara,
Most of the houses we are discussing are quite a bit smaller than yours. Clustering can often be done with minimal changes to the layout and gain efficiencies. Often times there will be 2 or more clusters, but when plumbing can stack vertically in walls and share common walls it does make plumbing, clearing clogs and repair more efficient.
 When plumbing is very spread out, it becomes very hard to maintain ¼” per foot of slope in horizontal waste pipes. As one example, if you have to connect 2 waste lines that are 40 ft apart you have 40/4” of drop or 10” now the pipe is 3.5” OD so you need 13 .5” of vertical space to do it in. If you have planned ahead and used a truss type floor joist that is 15” high you are home free. If you used a 11” I joist, you are screwed.
As far as bidding by the fixture, that depends on the plumber. The plumber that I used to use in remodeling situations bids by the complexity and what he has to do. At the same time I always show him the layout ahead of time and what I wanted to do before deciding on placement of fixtures, etc. Often he can tell me how the easiest (read cheapest) way to do something is. Sometimes moving a joist or 2 a few inches at the framing stage can make the plumbers job much easier.
 When I built my house, I had a plumber do the layout to be maximally efficient. The kitchen and bathroom layout were designed in part  around plumbing requirements. On the other hand, I knew I was doing the labor so it saved me time personally as well as materials costs. I worked it out so that the joist layout accommodated the plumbing layout and vice versa.
 
A GC I bicycle with and bounce questions off of  takes his lead framer, plumber, electrician and HVAC lead out to a good steak restaurant before they break ground on a new house he is building.   Afterwards they all go over the plans and each trade talks about what changes could be made to make their lives easier without affecting the overall design. When the plumber and HVAC guys know you are willing to make changes so that their job is easier/faster, they charge less. The framer can make the changes ahead of time and not have to come back in and fix the stuff the HVAC guy cut through. Using this type of systems approach pays big dividends.

Cheers
Eric

Eric, thank for that info which is actually helpful. I will take that advice. I have actually been trying to get input at the design stage from the subs but find I am in a catch 22. I don't know what subs I am using until I can get bids. I can't get bids until my plans are complete from the architect. I don't want the plans complete until I get input from the subs. I don't want to waste too much of a subs time if I am not sure I am using them, and I can't be sure until I get pricing.

But that is great advice about having everyone meet. I am really trying to keep everyone coordinated, which I think is probably the hardest part about building. My plans are now done from the architect, I will get bids, pick the guys I like and then if necessary make any changes to the plans.

Thanks again.

Posted By Alton on 23 Mar 2010 07:57 PM
Bidding by the fixture averages out from the plumber's point of view.  When it does not, the plumber raises the price per fixture for everyone.  Needless to say, it does not average out for the homeowner that clustered the plumbing.  Here in the South I gave up a long time ago on trying to save money by clustering the plumbing.

Alton;

you are right, unless you are dealing with a multi family or multi commercial bath clusters, plumbers do not give any consideration to fixture placements/savings