In our area most homes have basements with plumbing below the slab. Old septic drain fields are being replaced with mound systems. Sewage obviously doesn't flow uphill so gravity systems must be converted. Effluent, or ejector pump systems are the most common conversion.
@48windsor , I would be surprised if those systems are not commonly used in your area. I will caution you about system sizing. You are adding flow to an existing system. In our area, that is a touchy issue with existing septic. I have seen a number of remodeling projects that went out of whack because of older septic fields and new requirements.