I don't know what your local codes allow but here you would likely be required to put in an aerobic system rather than another conventional system with a drain field. Texas has become very stringent with their septic requirements requiring significantly larger drain fields than before. An engineer here will charge anywhere from $500 to over $1000 for the design of the system and overseeing its proper installation. The price depends on the type of system, how far from his/her office you are located, and any special requirements your state and county governments have.
The local average cost for a 3 bed, 2 bath septic install with engineer fees and permits is $8-10,000. More if working in rock or clay. Call a few septic installers and ask for a total price including engineering. They'll need to know your square footage and the number of bedrooms you have. Most should have someone they work and communicate well with. Then compare apples to apples in the bids as a total project. You'll save yourself some headaches of being a middleman between the engineer and installer down the line. Regardless, get the engineer's information including license number and phone number as part of the accepted bid just in case anything goes wrong. Also, get the installers license number and any insurance or bonding information he has for your records.
Todd Shell
Todd's Home Services
San Antonio, TX
