Does Williamson County, TX have its own adu rules ordinance?

No. Williamson County, TX does not appear to have a local adu rules ordinance, Texas state law applies instead. The rules summarized on this page come from Texas Statutes.

Key Facts Ordinance Text Full Breakdown Violations FAQ Sources More in Williamson County

Williamson County, TX Accessory Structures: ADU Rules (2026)

Few Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified September 2026

ADU rules in Williamson County, TX, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

Zoning: None, counties cannot zone
Size/Setback: No county limit
Septic Permit: Required if plumbed
HOAs: Often bar second dwellings

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Texas under recent ADU streamlining laws. The Williamson County, TX rules below cover local size, height, and setback requirements.

Summary

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.

City-specific rules exist: Cedar Park has its own adu rules rules that differ from Williamson County's county-level regulations. If you live in Cedar Park, check the city-specific page instead.

A person must hold a permit and an approved plan to construct, alter, repair, extend, or operate an on-site sewage disposal system.

Source: Texas StatutesView official code

Full Breakdown

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.

Violations & Fines

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.

Frequently Asked Questions

Can I build an ADU in unincorporated Williamson County?

The county cannot zone it out. If it has plumbing, you need an OSSF septic permit under Chapter 366, and HOA deed restrictions may prohibit a second dwelling.

Does an ADU need its own septic?

The septic must be sized for the added bedrooms. Section 366.051 requires a permit and approved plan to construct or alter the on-site sewage system.

Sources & Official References

[1]Texas Statutes

Other rules in Williamson County

Animal Ordinances: Dog Leash LawsFew Restrictions Animal Ordinances: Chickens & LivestockFew Restrictions Garage ConversionsLight Restrictions Tiny HomesFew Restrictions Animal Ordinances: Exotic PetsLight Restrictions Animal Ordinances: Breed RestrictionsFew Restrictions Landscaping Rules: Artificial TurfFew Restrictions Landscaping Rules: Native PlantsFew Restrictions

All Williamson County rules

How Williamson County compares: Easiest Cities to Build an ADU·Texas rules heatmap·Compare Williamson County to another location·View the Texas accessory structures overview

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ADU Rules in Cities Across Williamson County

Round Rock, TX Cedar Park, TX