I have read every answer on the thread, and I don't want to be one of those OPs who ask for opinions, and then say, "No, you're all wrong!" or come out with a massive drip-feed which changes the whole thing: "Yes, but we have to sleep in a tent because he has our bedroom!" (this is not the case)
When he moved in, we were so poor we had less coming in than our very basic outgoings. We have now sold our house, moved and downsized, hence the need for sheds. We got rid of a lot, but then my Mum died and I had to store a lot of her stuff until I could face sorting and getting rid of it.
We have cleared out more stuff now, but DH is disabled and now has a mobility scooter which needs to be stored in one of the sheds, so they're full again! And we have only a tiny loft, which is about 3 ft high.
No doubt those calling me tight would happily pay £689 for their adult son to store his collection of Star Wars toys, a million DVDs and other crap, just because he once had room for them. The shed is 6'x8' and is crammed floor to ceiling already - almost another roomful of stuff. I think it was very kind of us not to say, "Well, it's your stuff, love, you'll have to cram it in your room somehow or sell it." I don't think it was petty to expect him to pay for storage of his own property. But I take your point about overpaying for the shed. We won't charge him, once it's paid for.
With regard to supporting our son financially because he has MH problems, if I thought it would fix him, I would. Unfortunately, it would just mean he spent an extra £200 a month on toys, and eventually 'needed' another shed, and another...
But, worse than that, why would he ever leave, when a double room anywhere else is already £100 a week, and here, he has the use of an entire house, no food or washing or heating to pay for? At the moment, he has no incentive to leave. His friend has MH and gets benefits, rents a flat and has to pay more than £50 extra on top of the Housing Benefit he receives, plus bills.
We don't need the money now, we could afford to keep him free, but I thought if we gradually ask him to pay more, having his own place would seem more appealing. We don't want to support him for the next 25 years, die of old age and leave him at 63 so fucking useless at dealing with life that he has to go into care. He has enough money to pay for therapy, but would rather buy collectibles and block out reality.