According to NAHB analysis of the Survey of Construction (SOC) data, only 8 percent of new single-family homes started in 2013 are served by individual wells and only 15 percent have private septic systems. These shares, however, vary widely across the nine Census divisions and reach 42 and 51 percent, respectively, in New England.
The SOC which is conducted by the US Census Bureau and partially funded by the Department of Housing and Urban Development (HUD) collects detailed information on physical and financial characteristics of newly-built single-family homes. The information comes from interviews of builders or owners of the selected new houses. About 1 in 50 new single-family homes are selected for interviews based on a sample of building permits and a canvassing of areas not requiring permits. This sample is large enough to provide estimates for nine Census divisions.
The SOC classifies sewage disposal systems as public sewers (including community or shared sewage/septic systems) and individual septic systems. Most of new single-family homes are serviced by public sewers. The incidence of individual septic systems among new single-family starts varies by region.
More than half of new single-family homes started in New England have private septic systems, while the national share is slightly above 15 percent. Individual septic systems are also more noticeable in the East South Central and East North Central division with the corresponding shares of 36 and 28 percent. The share of private septic systems is also above the national average in the Middle Atlantic division where it reaches 19%.
The common sources of water supply also differ noticeably by geographic location. Similarly to sewage/septic systems, the SOC classifies community or shared water supply/well as public and not individual wells. Nationally, only 8 percent of new single-family homes started in 2013 are served by individual wells, and the remaining vast majority of new homes are served by a public water system.
In New England, where new homes sit on nation’s largest lots and are more likely to be custom-built, 42 percent of new single-family homes are built on individual wells. The reliance on private wells is also relatively common in the East North Central and Middle Atlantic divisions where the corresponding shares are 27 and 18 percent.
In contrast, individual wells are almost non-existent in the West South Central and East South Central divisions where their shares are less than 2 and 3 percent, respectively.
The numbers for the mountain region might be inaccurate. I am guessing you are only looking at new start permits reported. A good part of the rocky mt region development is taking place outside of cities, in many areas permits permits aren’t required. For example, over 60% of new home in Montana were built on wells last year, all of which occurred outside of municipalities, so they don’t show up on permit reports.
The Census Bureau does adjust the Survey of Construction (SOC) sample to include new homes built in areas not requiring building permits. As a matter of fact, the SOC consists of two parts: the Survey of Use of Permits (SUP), which looks at new construction in areas that require a building permit, and the Non-Permit Survey (NP), which focuses on new construction in areas that do not require a building permit.
I suspect the discrepancies come from how INDIVIDUAL wells are defined. If a particular well is shared by a community then the Census Bureau refers to it as public (community/shared) water supply rather than an individual well.