From comments made by our Architect and GC when we were discussing alternatives that would to stack two bathrooms, Bry seems to be correct that the savings are indeed minimal. Both said it really didn't make much difference. Our SIL (master Plumber and HVAC guy) said the same and he is is going to be the one doing the work. Of course significance is relative, so maybe in builder level or lower cost homes, the difference might be 'significant" and certainly it is an an oft quoted "rule" on this forum that there are big savings in consolidating plumbing, but our pros don't seem to agree that any savings would be significant enough to warrant design changes just to consolidate.

Our builder also indicated that there was not a meaningful difference. What did make a difference cost-wise was materials -- our area is behind the times and does not allow "plastic plumbing", even for vents. In our case more thought was given to water conservation and energy efficiency in water heater planning and placement (and use of recirculating pump).

Careful plumbing location is far more important for multifamily buildings and hotels or subdivisions where a house design will be used repeatedly.

The savings from locating bathrooms near other bathrooms is not as important as the savings in structure and the benefit of a good layout. If there is a basement or crawl space, plumbing can pretty much be located anywhere but it does help to anticipate where waste pipes will be located in the framing of the second floor in order to avoid soffits.

A more important issue is to avoid locating plumbing next to or above living spaces where noise would not be welcome and to avoid exposure to outside freezing temperatures.

Figure out the big issue before getting side tracked by the little ones.

The spacial relationships is the 'dog' and the plumbing is the 'tail'. Some layouts in space may save in plumbing costs, but the plumbing should not drive the layout to the point where it compromises the functional layout. Unfortunately weighing space relationships vs. plumbing costs is difficult due to one is convenience and the other is dollars. Don't forget to factor in the time and effort it takes to try to figure it out.

I also asked our builder about cost savings in stacking water sources and he said basically what bry, mojo, and oaktown said. Cost savings isn't enough to justify worrying about it.

Copper and iron are required in our town. We didn't have to increase the diameter of the teardown's water supply line by sticking to 3.1 bathrooms (more than adequate for us).

One of my favorite things in our house is the recirculating hot water. We have natural gas and stuck with tanks; is anyone still doing that instead of tankless?

@ chisue - Where do you live that cast iron is required? I assume that you are also allowed to use PVC hookups and vents just a cast iron stack. Is that correct? Just curious - that seems a bit odd (ridiculous) to require it in residential single family dwellings.

I get that plumbing building cost savings aren't significant, but sometimes people mention point-of-use hot water heaters under bathroom sinks - would that make a difference in planning bathrooms back-to-back, or close enough to use a single 'local' hot water heater? Forgive my ignorance on the details - I had assumed from the way they are discussed that they save on overall water usage by obviating the need for or shortening running the water until it's hot. Thoughts?

I'd worry about a lot of other things first--site planning and orientation, utility access, floor plan and exterior massing, etc., before I'd start focusing on plumbing. If you have all these other things covered, then good on you. If not, you may want to shift your focus...

Thank you all for your insights. It sounds like the consensus is that:

- Taking an intentional step to locate all rooms requiring plumbing close to each other for the sole purpose of reducing build cost is not advisable.

- Care should be taken in the designs to understand where drain pipes could/will be run (for instance avoiding through a wall in a common area where you will hear each flush).

- Care should be taken in understanding where supply pipes could/will be located in those zones where freezing is a concern.

- It is advisable to keep some distance (make the runs longer) between the central drain and bathrooms, shorter between the central drain and kitchen sinks.