If your proposed roof intrudes into the common property airspace (2.44 metres above the floor) then you have to pay the owners corp compensation. The calculations for that (as established in the High Court) is the estimated or agreed difference between the value of the property before the addition and after it, minus all the costs of the work, including legals, archistects fees, DA submissions and building work.
In other words, if you increase the value of your propery by $50k and it costs you $30k to build, with all legals etc included, you should offer the owners corp $20K.
You will get the use of the improved property, plus a share of the value when you money drops into your Admin fund.