Why is this a problem?
RC-120 has safeguards to protect Tidal Wetlands from damage that could occur by excavating, filling, and grading. It specifies a 100 ft non-infringement zone from Tidal Wetlands. The RA-15 zone lacks such safeguards and permits activity the could harm or destroy Tidal Wetland. This creates a loophole that somebody could use to build a huge dock and a driveway on a Wetlands property. This is exactly what is being done on 28 Schoolhouse!

Here is an excerpt of a letter I sent to the Stonington Zoning and Planning Dept

 In 1979, some participants in the zoning discussion wanted to maintain the 15,000 sq ft requirement in effect for the group of 4 properties on Schoolhouse Rd as opposed to the much larger requirement of RC-120. As you note, three of the properties were already developed, and would become non-conforming since RC-120 requires 130,000 sq ft. The homes had been there for a very long time, 30 to 100 years so an exception could easily have been made at the time. The 28 Schoolhouse property was quite different from the other properties. It was never developed. At the highest tides it was essentially surrounded by water and it consisted of at least 50% Tidal Wetland; 28 Schoolhouse is exactly what the RC-120 designation describes:

"This zone encompasses coastal areas including coves, estuaries, tidal marshes, and wetlands. It is established to preserve the natural habitat and assure flood protection."

Aerial photos demonstrate the undeniable fact that the property resembles other RC-120 properties and not RA-15. It was undeveloped then and remains so 40 years later. In 1979, the 28 Schoolhouse property should have been zoned for what it was, RC-120. You can call it residential, but it is still wetland.

Here is some documentation comparing zones RA-15 and RC-120. Based on the Stonington Zoning Regulations
RA-15 and RA-20 = "These zones are for areas that have been previously developed as single family house lots and which should be maintained as such to preserve the character of the Town." Stonington Zoning 20183.4  

This Google map area of Pawcatuck is an example RA-15. Lots of houses, no Tidal Wetland!

RC-120 = "This zone encompasses coastal areas including coves, estuaries, tidal marshes and wetlands. It is established to preserve the natural habitat and assure flood protection. Non-infringement area of 100 feet minimum from tidal marsh and significant natural resources such as, but not limited to, inland wetlands, estuary shoreline and bodies of water" Stonington Zoning 3.1

The area in the pic below shaded green is RC-120 while the part in red is RA-15

Lets take a closer look at the property that the borough is calling RA-15, circled in red 

And a closer view with high tide in light blue and the extent of a very high tide in dark blue. 

Who can honestly believe that this is not a costal wetlands property?