I own several vacant lots and also flip lots on the side. Some I bought very cheap and am waiting for sewer.  I'm considering putting a mound system (septic) on a particular lot that I'm making an offer on. The cost figures run between 10-20K with annual costs of about $500 for maintenance,  Has anyone here been successful using mound systems for flips or for buy, build, rent and hold?  They seem to be used most often for custom built homes.