Did your parents know or did you have to hide it from them?
Mum (& dad, but didn't live with him) knew I drank.
Neither knew I smoked til I was 21 (started at 16), still didn't smoke in front of them. Did ONCE in front of my mum cos we were away for the weekend and she clocked why I was being antsy after 24 hours and told me just to have a cigarette, felt like murdering someone in front of a police officer ha.
Didn't take drugs (except smoked weed occasionally but it doesn't agree with me) til I moved out, but my mum knows about that now.
Did they prefer you drank/smoked/took drugs indoors so they could be there if you needed them/supervise you or did they forbid you from doing this under their roof?
They weren't especially bothered either way - I rarely had friends over as we lived in an awkward place miles away from where all my friends lived, but if I did have friends over we could drink a sensible amount and that was fine.
Did your friends also smoke/drink/take drugs at your house, or did you go to your friends houses?
See above, but mostly was at my friends' houses. Their parents all let us drink there from 14+ - the odd Barcardia Breezer younger, up to pretty much whatever we wanted if not being sick at about 17.
How long did you drink/smoke/take drugs for?
Still drink (occasionally).
Stopped smoking at 25.
Took drugs semi-regularly (not as in, an addiction, but at festivals/big nights out 4-5 times a year) for about a decade (18-28) and a very small handful of times in the 5 years since.
My parents were teens in the 80s and took a fair amount of drugs themselves. Their message was always, we'd really rather you didn't but if you do, please be careful about dosages, who you're with, and you can call me at any time, no matter how old you are, and I'll be there if you need me. It didn't seem glamorous or forbidden, they were quite matter of fact. A friend's mum, who was very open about being a raver with a penchant for pills, took an almost-encouraging approach, letting the kids get weed delivered to the house while she was there, and both her kids had quite bad active addictions by 21, so I think you can be too permissive.