Rainwater Harvesting - Rules, Regulations, and Restrictions by state.

Below is a composition of laws, regulations, and restrictions on citizens' ability to collect and store rainwater for gardening, irrigation, and consumption. 
This is by no means an exhaustive guide. The onus is on the individual to conduct their own thorough research for their area and intent. We've focused on state level guidance but local districts may impose their own regulations. If you're planning or already practicing rainwater collection, it might help to further research.
Also, after this guide we'll discuss the basics of collecting and storing water.
》Alabama
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Alaska
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Arizona
Regulated: Yes. Check county regulations. Some incentives may be offered to residents.
Restrictions: Some building restrictions apply to new construction. Check local regulations for specific information.
Permit required: Some counties require certifications for installations. 
Quantity: Check local regulations
Reference: https://new.azwater.gov/conservation/landscaping
》Arkansas
Regulated: Yes
Restrictions: The State Board of Health shall allow the use of a harvested rainwater system used for a non-potable purpose if the harvested rainwater system: (1) Is designed by a professional engineer licensed in Arkansas; (2) Is designed with appropriate cross-connection safeguards; and (3) Complies with the Arkansas Plumbing Code.
Permit required: Yes
Quantity: N/A
Reference: https://law.justia.com/codes/arkansas/2017/title-17/subtitle-2/chapter-38/subchapter-2/section-17-38-201/
》California
Regulated: Yes.
Restrictions: Varies by use case.
Permit required: At the state level, no. But check by county.
Quantity: The state level does not regulate a quantity. Check county rules.
Reference: https://bayareaanswers.com/is-it-illegal-to-collect-rainwater-in-california/
https://leginfo.legislature.ca.gov/faces/codes_displayText.xhtml?lawCode=WAT&division=6.&title=&part=2.4.&chapter=&article=
https://leginfo.legislature.ca.gov/faces/billTextClient.xhtml?bill_id=201120120AB1750
》Colorado
Regulated: Yes
Restrictions: Yes
Permit required: 
Quantity: 110 gallons for outdoor use only. A residence that relies on a well may collect an unlimited amount for indoor/outdoor use.
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Connecticut
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Delaware
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Florida
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Georgia
Regulated: Yes
Restrictions: Regulations are in place allowing rainwater harvesting; however, no specific technical resources or incentives are provided
Permit required: N/A
Quantity: The state level does not regulate a quantity. Check county rules.
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Hawaii
Regulated: Yes
Restrictions: Check local guidance
Permit required: N/A
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Idaho
Regulated: Yes
Restrictions: Regulations are in place allowing rainwater harvesting; however, no specific technical resources or incentives are provided
Permit required: Yes
Quantity: N/A
Reference: https://www.harvesth2o.com/adobe_files/Idaho_Ltr%20Sen%20Schroeder%20081108.pdf
》Illinois
Regulated: Yes
Restrictions: Requires rainwater harvesting collection systems and rainwater harvesting distribution systems to be (A) used only for non-potable uses and (B) constructed in accordance with the Illinois Plumbing Code. Defines "rainwater harvesting collection system" and "rainwater harvesting distribution system"
Permit required: Check local guidance.
Quantity: Rain barrels/cisterns containing more than 55 gallons must have proper structural foundations.
Reference:https://www.chicago.gov/content/dam/city/depts/water/general/Engineering/SewerConstStormReq/2016StormwaterRegulations.pdf
》Indiana
Regulated: No
Restrictions: No statewide rainwater harvesting regulations; check county for local incentives
Permit required: Check local guidance
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Iowa
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Kansas
Regulated: Yes
Restrictions: Yes
Permit required: Yes, check local regulations
Quantity: Check local regulations
Reference: https://agriculture.ks.gov/divisions-programs/dwr/water-appropriation/water-law-basics
》Kentucky
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Louisiana
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Maine
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Maryland
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Massachusetts
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Michigan
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Minnesota
Regulated: Yes
Restrictions: Regulated through the Minnesota Plumbing Board, which requires permitting and code compliance; check for local incentives
Permit required: Check local guidance
Quantity: n/a
Reference: https://stormwater.pca.state.mn.us/index.php/Design_criteria_for_stormwater_and_rainwater_harvest_and_use/reuse#Identify_required_permits_and_applicable_codes
》Mississippi
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Missouri
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Montana
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Nebraska
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Nevada
Regulated: No
Restrictions: Check county requirements
Permit required: Check county requirements
Quantity: 20,000 gallons
Reference: https://www.leg.state.nv.us/NRS/NRS-533.html#NRS533Sec070
》New Hampshire
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》New Jersey 
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》New Mexico
Regulated: Yes
Restrictions: Yes. Outdoor use, no regulations. Indoor, check county requirements
Permit required: Check county requirements
Quantity: Check county requirements
Reference: https://www.ose.state.nm.us//WUC/wuc_rainHarvesting.php
》New York
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》North Carolina 
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》North Dakota
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Ohio
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: http://codes.ohio.gov/orc/3701.344
》Oklahoma
Regulated: Yes
Restrictions: Check local guidance
Permit required: Check local guidance
Quantity: Check local guidance
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Oregon
Regulated: Yes.
Restrictions: Location/use dependent
Permit required: Check local/use
Quantity: Check with county regulations.
Reference: https://www.oregon.gov/bcd/documents/brochures/3660.pdf
https://www.oregon.gov/bcd/codes-stand/Pages/plumbing.aspx
》Pennsylvania
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Rhode Island
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》South Carolina
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference:
https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》South Dakota
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: none
》Tennessee
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Texas
Regulated: Yes
Restrictions: Plans for new home construction require permitting if rainwater harvesting is to be used.
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Utah
Regulated: Yes
Restrictions: Yes
Permit required: <100 gallons, No. Anything over, yes. 
Quantity: 2,500 gallons
Reference: https://www.waterrights.utah.gov/forms/rainwater.asp
》Vermont
Regulated: Yes
Restrictions: Check local guidance
Permit required: Check local guidance
Quantity: Check local guidance.
Reference:https://dec.vermont.gov/sites/dec/files/documents/VermontStormwaterManagementManualRule_Ch36_2017_Final_2016-12-20.pdf
》Virginia
Regulated: Yes
Restrictions: Check local guidance
Permit required: Check local guidance
Quantity: Check local guidance.
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Washington
Regulated: Yes. However, the state may offer stormwater fee incentives for the collection of rainwater.
Restrictions: Varies based on local/use.
Permit required: For specific applications, and county by county basis.
Quantity: Varies depending on use case.
Reference: https://ecology.wa.gov/Water-Shorelines/Water-supply/Water-recovery-solutions/Rainwater-collection
》West Virginia
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Wisconsin
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
》Wyoming
Regulated: No
Restrictions: No
Permit required: No
Quantity: N/A
Reference: https://www.energy.gov/eere/femp/rainwater-harvesting-regulations-map
Sources (other): https://watershedcommittee.org/wp-content/uploads/2013/06/Rainwater-Manual.pdf | https://new.azwater.gov/sites/default/files/media/Harvesting%20Rainwater_0.pdf
RAINWATER BASICS
  1. The first step is to determine the collection area. The collection area can be any flat or sloping surface that can catch rainwater such as rooftops or home-made funnels systems.

  2. Since gutters and downspouts are most commonly used, we'd recommend this for beginners. Use existing roof gutters to collect water then use downspouts that lead to a storage container. There are advance options you could consider such as a "first-flush" system that diverts the first few gallons away from your cistern so that less airborne pollutants are collected. Consider adding multiple collection downspouts and cisterns/barrels to increase your collection.

  3. Choose a container to store the collected rainwater. The container should be made of a food-grade material such as plastic. There are pretty cheap (or free) containers if you search in online marketplaces (we found a 275 gallon tank that use to old sucrose syrup). It should have a tight-fitting lid to prevent debris and insects from entering. Consider installing your cistern/barrel on an elevated platform to improve water pressure. You can also add a pump so that you can elevate the waterflow up to a sink or shower. They're fairly cheap, we used both a 1.5gpm and 7gpm pump and found 1.5 was all we needed. We also hooked ours to a propane water heater. It is powered by a bank of marine deepcycle batteries.

  4. Install a filter at the downspout to prevent leaves, debris, and other contaminants from entering the storage container. Plumbing experience is helpful but if you're just looking to collect non-potable water (like for your garden) then really anything will work if you can divert water into your container. Clean out your rain barrel or cistern regularly to prevent debris and insects from accumulating. A bleach additive or irriator may help prevent residue build up and longevity of your water.

  5. Install an overflow system to divert excess water away from the storage container in case of heavy rainfall or infrequent usage. 

  6. Use the collected water for non-potable purposes such as watering plants, washing cars, or flushing toilets. Add a filter beyond the outlet if you plan on drinking the water. Of simply filter it yourself, then boil it or treat it with purification additives. Depending on your system you bay be able to drink it even after a rudimentary filtration.