Thornton, CO Cannabis Regulations: Home Cultivation (2026)

Thornton's Section 38-298 imposes ten location, size, and safety restrictions on home marijuana cultivation, adopted by Ordinance No. 3341 in 2015 and last amended by Ordinance No. 3745 in 2025. Location: marijuana "may only be cultivated within a person's primary residence or detached accessory structure on the lot where the primary residence is located," and only on a lot with a detached dwelling or manufactured home; cultivation is barred in "any garage, whether attached or detached, any kitchen, bathroom, or any room concurrently used for the purpose of sleeping," and "shall not be cultivated outdoors" under any circumstance.

Plant limits: no more than six plants per qualifying adult, and "in no event shall more than 12 marijuana plants of any size be cultivated anywhere on the lot," no matter how many adults or medical patients live there. " Security: the grow must sit in an "enclosed locked space," defined as an area with solid walls and roof secured "by combination lock or lock and key," and a hoop house or greenhouse does not qualify. Lighting is restricted to "light emitting diodes (LED), compact fluorescent lamps (CFL) or fluorescent lighting"; all high-intensity discharge lighting, including metal-halide, sodium-vapor, and mercury-vapor lamps, is prohibited.

Growers cannot store cultivation chemicals "inside any dwelling unit," must ventilate and filter the grow so odors aren't "detectible beyond the property line," and cannot let light, glare, or smell interfere with a neighbor's use of their property. " Registered medical marijuana patients get two affirmative defenses: possessing a valid state registry card excuses growing up to 12 plants (rather than six), and excuses a caregiver or cardholder from the locked-space access restriction for minors.