Sandy shorelines, great views, and clean water go a long ways. 

Leach fields are sized on how the soil perc tested and the size of the tank.

For sewer, if there are no sanitary sewer lines, which is common in the country, you will need to have the county health department come and test the soil for a "perc test" that will tell you if your soil is suitable for leach fields that will slowly decontaminate wastewater from your home.

Then on Amazon, look for books by Diane Kennedy, Sandy Botkin, Stephen Fishman. 

On the Oregon Coast, spots like Cannon Beach, Lincoln City, and Manzanita mix sandy beaches and cliffs, with zoning rules often allowing detached guest houses and ADUs.

Hi Sandy,I can recommend Cris Gottlieb at Cohen Hurkin.

Then I backfill with gravel to help with drainage top with soils and plant seed.   

Assuming there are no other issues like soil, etc that would prevent this type of addition, does anyone have any experience in this, and any ballpark estimates for cost?

You might be sitting on land for a year waiting for permits, soil reports, inspections, etc. etc. before being cleared to build.

I have read much about Radon, and it really doesn't end on just the soil content, as it could also be in the water..