Autism

Yeah. It's a lot easier to slip out without saying anything, and apologize after if they ask, than it is to risk your appointment being missed.

If you can, just walk out. If you know they're going to ask (after or before) have a quick, simple, and mostly true lie prepared. You're going to grab lunch real quick since you've been busy all day with the basement, you are headed out to get useful stuff for the basement and do they have any ideas for anything specific, etc. Bonus points for actually also doing the thing you said you were doing.

Maybe say you need to buy a specific thing against the flooding (maybe buckets or a pump or something like that idk) and then do both your appointment and getting that thing.

If not I still wish you got look with getting back up to speed on your mental health:)

Help, then when it's time, go for your appointment, say you'll be back soon (give them a time if need be), then do it.

Help as much as you can stand to, but do NOT miss that appointment. Lie your ass off if you have to, but get that diagnosis, trust me. It is very much worth it, and you'll be able to get the help you need right away as an adult.

My first rule in life, look after yourself. You can't help others properly if you don't care for yourself.

If it's a choice between pissing off my parents because I leave when they want help, and missing medical help in a crowded system? I'd go for upsetting parents, they'll get over it.

Look after yourself, go get help. This is coming from someone (me) who has mental health issues. Your future self will thank you if you start the path of treatment sooner rather than later.

Hope you're feeling well now. Take care!

First, remember that there may be other opportunities to get diagnosed, so this isn’t necessarily the end of the world.

Second, your mental health is a lifelong concern, while a flooded basement is temporary. You can help out before and after your appointment, and realistically, your contribution might not be crucial. If you're young enough to need your parents' consent for healthcare, then you're also young enough that dealing with the mess in the basement shouldn't fall entirely on you.

Additionally, if they aren't willing to support you when you need help, it's understandable to question why you should be expected to drop everything when they need assistance. That might be my oppositional defiant disorder talking, or it could just be that I don't agree with parents who think they can do no wrong, and therefore you don't need mental health care.

Btw, how old are you?

Is it possible to just tell them it's a medical appointment, but it's about something that you don't feel comfortable talking about?

It's the truth, it explains the secrecy about the appointment, but it keeps the secret.