Not only is it ill advised to allow teens to use marijuana (or alcohol or other drugs) at home but it is also illegal now in many Marin jurisdictions. Local Social Host Ordinances in Marin have existed for over a decade and hold adult homeowners or renters liable for fines of $750 or more for underage parties or gatherings involving alcohol.
Now, because of the anticipated harms to adolescents with the legalization of marijuana, many of those ordinances have been amended to specifically include marijuana and other controlled substances in addition to alcohol. Also specifically covered in those jurisdictions are rental facilities, party buses and limousines. In addition to payment of a fine, those issued a social host ordinance citation can be required to attend a restorative justice program such as Marin County's YMCA Youth Court which among other things, requires attendance by teens at youth court sessions, and with their parents, at a six-hour Saturday class.
Adults are strictly responsible whether they are at home or at the rented facility, bus or limo and whether they have knowledge or not of the gathering, alcohol or other drugs. And liability may result if as few as two or more minors (the number depends on the jurisdiction) are present consuming alcohol, marijuana or other drugs.