In Texas, no permit is required to use a greywater system when the flow is less than 400 gallons per day.

Regulatory overview:

Texas Health & Safety Code - Section 341.039. Graywater Standards

§ 341.039. GRAYWATER STANDARDS.

(a) The commission by rule shall adopt and implement minimum standards for the use and reuse of graywater for:

(1) irrigation and other agricultural purposes;
(2) domestic use, to the extent consistent with Subsection (c);
(3) commercial purposes; and
(4) industrial purposes.

(b) The standards adopted by the commission under Subsection (a) must assure that the use of graywater is not a nuisance and does not damage the quality of surface water and groundwater in this state.

(c) The commission may not require a permit for the domestic use of less than 400 gallons of graywater each day if the graywater:

(1) originates from a private residence;
(2) is used by the occupants of that residence for gardening, composting, or landscaping at the residence;
(3) is collected using a system that overflows into a sewage collection or on-site wastewater treatment and disposal system;
(4) is stored in tanks that:

(A) are clearly labeled as nonpotable water;
(B) restrict access, especially to children; and
(C) eliminate habitat for mosquitoes and other vectors;

(5) uses piping clearly identified as a nonpotable water conduit, including identification through the use of purple pipe, purple tape, or similar markings;
(6) is generated without the formation of ponds or pools of graywater;
(7) does not create runoff across the property lines or onto any paved surface; and
(8) is distributed by a surface or subsurface system that does not spray into the air.

(d) Each builder is encouraged to:

(1) install plumbing in new housing in a manner that provides the capacity to collect graywater from all allowable sources; and
(2) design and install a subsurface graywater system around the foundation of new housing in a way that minimizes foundation movement or cracking.

(e) In this section, "graywater" means wastewater from clothes-washing machines, showers, bathtubs, hand-washing lavatories, and sinks that are not used for disposal of hazardous or toxic ingredients. The term does not include wastewater:

(1) that has come in contact with toilet waste;
(2) from the washing of material, including diapers, soiled with human excreta; or
(3) from sinks used for food preparation or disposal.

More details:

The full version of the Texas greywater regulation can be found here.