A.Jim Stack, a plumbing contractor in Kirkland, Wash., responds: The drain-line requirements in today's plumbing codes were developed decades ago, and were based on the number of fixtures needed to reliably clean and scour lines of various sizes. At the time, toilets used 3.5 to 4 gallons per flush.
Fast-forward to today, when water use has been reduced by more than half, to an average of 1.6 gallons per flush. The newest generation of high-efficiency toilets, or HETs, use just 1.28 gallons per flush. Most of these fixtures flush well enough to clear the bowl with that amount of water - but once the waste leaves the fixture itself, the lowered flows are not always adequate to carry it along drain lines sized for much larger volumes. I've seen a sharp increase in main-line stoppages in the past 15 or so years, and expect to see more as HETs come into wider use.
It can be helpful to direct the discharge from the clothes washer - which, during its drain cycle, generates the largest outflow of any household fixture - to where it will best contribute to flushing the drain lines. There are two schools of thought on how to do this. One approach is to put the washer discharge as far from where the drain exits the house as possible, allowing it to regularly flush the indoor line of any accumulated solids. The other is to put the washer discharge very near the exit, so the force of the discharge helps clean the drain line to the street.
I generally prefer the second approach, especially in older homes where solids tend to accumulate in rough-surfaced older pipe. Also, before starting a remodeling project, you might want to use a camera to inspect the building drain, especially if there have been problems in the past. That way, both you and the homeowner will know what you may be dealing with in the future.