ADU rules in Williamson County, TX — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
Texas counties cannot zone, so unincorporated Williamson County sets no ADU size, setback, or occupancy limit. An accessory dwelling with plumbing requires an on-site sewage (OSSF) permit under Health & Safety Code Chapter 366.
Williamson County cannot zone, so it imposes no accessory dwelling unit standards, no owner-occupancy rule, and no size cap in the unincorporated county. The real constraints are septic capacity and deed restrictions. Any ADU with a kitchen or bathroom adds wastewater load, and Health & Safety Code Section 366.051 requires a permit and approved plan to construct or alter the on-site sewage disposal system serving it, issued by Williamson County as the authorized agent. The existing septic system must be sized for the added bedrooms or a new system installed. HOA deed restrictions in subdivisions frequently prohibit second dwellings.
Adding an ADU on septic without an OSSF permit violates Chapter 366 and stops the project until permitted. HOA deed restrictions barring second units are enforced by the association.
Other ordinances people look up for this city. Green dot = verified primary-source excerpt.
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