New rules in the past ten years or so have made designing an onsite waste water system a bit of a science. Here are some general things I would consider if designing a system for you. Using your house plans the number of bedrooms, bathrooms and people in the home would be considered. If you design the house with a future suite in the basement it would be considered as well. If you are installing a large soaker tub or a garborator they would need to be factored into the overall design. Generally you need to design the system based upon a maximum water usage sinario. This means you will need a system that can handle a worse case full load over a sustained period of time. Once you realize the added costs to having that large home with 8 bathrooms you may want to reconsider. The system must be designed for max peak even though the average flow is going to only be a small percentage of that.