Your question caused the hair on the back of my neck to rise. I have been a county counsel in two Western states, where my duties included pursuing public nuisance actions against individuals who installed their own septic systems without complying with local health or building regulations. Because septic systems are now required to treat waste water rather than merely dispose of it, most states heavily regulate installation. I have seen many instances where a landowner destroyed with an illegal installation the only suitable spot on their land which could have supported a legal system; the result is a non-buildable lot. Check your local regulations; a self-installed systme may be penny-wise, but dollar foolish.