Does a small home or ADU need a smaller septic system?

It can, but there is a floor. In Texas, the smallest residential category in Table III covers homes with 1 to 2 bedrooms and up to 1,500 sq ft, rated at 180 gallons per day with water-saving fixtures or 225 without. A 400 or 800 sq ft one-bedroom home falls in that bottom category, so it is not sized smaller than a typical two-bedroom. The design still has to fit the soil, setbacks and lot size rules, and an aerobic or drip system may be needed on tight or poor sites. If the small home is an extra dwelling on a lot that already has a house, see the rules on connecting a second dwelling.

Check your local rule. This varies by state, county or city. Your permitting office has the final word; the official lookup pages below show you who that is.

Sources

• 30 TAC §285.91(3), Table III: Wastewater Usage RateTexas Administrative Code (figure via Justia Regulations)
• Types of Septic SystemsU.S. Environmental Protection Agency