First and foremost, hire a good septic designer to do the necessary tests and drainfield design , BEFORE you design the house!!! Go out with them and tell them where you would like to situate the house, but let them give their input. Sometimes just moving the house a few feet in one direction will make the difference between a regular, gravity fed system and a more expensive pump system.
I had some friends that were going to build and I tried advising them on this, but wouldnt listen. The irony is, if they would have listened and moved the house about 10 feet to the west, they could have gone with a gravity fed system for about $3,500. As it turned out, they spent more than $8,000 because they put the house right where the drainfield should have gone.
Also, if they say you cannot go with a regular septic system design, ask them about ALL the different types of systems you may be able to go with and the advantages/disadvantages of each type. If they are pushing you toward one type of septic system design it may be because that is the only type of system they do. You may want to take your copy of the site/soil conditions (make sure to get a copy) and call/visit...