Years ago someone in my block bought an unused common property plant room that was only acessible from their lot – and was therefore of little use to anyone else – and turned it into a wine cellar.
If I were you, I would get two or three local real estate agents to tell you what they thought it was worth to the owners corp as is, and what value it would add to your property and pitch the purchase offer somewhere between the averages of the two.
Remeber, sale of common property requires a special resolution – 75 percent of people voting at a meeting – so you have to convince owners who are actively engaged (or they wouldn’t be at the meeting). And you may have to pay for the general meeting if there isn’t an AGM scheduled for any time soon.